diff --git a/copier.yml b/copier.yml
index 977b2b4f..52d76193 100644
--- a/copier.yml
+++ b/copier.yml
@@ -127,6 +127,11 @@ central_infra_github_organization_name:
     type: str
     help: What is the name of the GitHub organization that owns the central infrastructure repository?
 
+central_infra_github_organization_id:
+    type: int
+    help: What is the numeric ID of that GitHub organization? Open https://api.github.com/orgs/<org-name> in a browser (no login needed) and read the "id" field, or run `gh api orgs/<org-name> --jq .id`.
+    validator: "{% if central_infra_github_organization_id <= 0 %}Must be a positive integer{% endif %}"
+
 initial_iac_management_deploy_occurred:
     type: bool
     help: Has the initial merge to main and the deployment of the IaC Management stack occurred yet?
diff --git a/template/src/aws_central_infrastructure/iac_management/lib/constants.py.jinja b/template/src/aws_central_infrastructure/iac_management/lib/constants.py.jinja
index af38d288..cf260c86 100644
--- a/template/src/aws_central_infrastructure/iac_management/lib/constants.py.jinja
+++ b/template/src/aws_central_infrastructure/iac_management/lib/constants.py.jinja
@@ -3,4 +3,5 @@ CLOUD_COURIER_INFRA_REPO_NAME = "{% endraw %}{{ cloud_courier_infra_repo_name }}
 CENTRAL_INFRA_PROD_ACCOUNT_ID = "{% endraw %}{{ aws_production_account_id }}{% raw %}"
 IDENTITY_CENTER_ACCOUNT_ID = "{% endraw %}{{ identity_center_production_account_id }}{% raw %}"
 CENTRAL_INFRA_GITHUB_ORG_NAME = "{% endraw %}{{ central_infra_github_organization_name }}{% raw %}"
+GITHUB_ORG_IDS = {CENTRAL_INFRA_GITHUB_ORG_NAME: {% endraw %}{{ central_infra_github_organization_id }}{% raw %}}
 CONFIGURE_CLOUD_COURIER = {% endraw %}{{ 'True' if configure_cloud_courier else 'False' }}
diff --git a/template/src/aws_central_infrastructure/iac_management/lib/github_oidc_lib.py b/template/src/aws_central_infrastructure/iac_management/lib/github_oidc_lib.py
index ff8609ad..08ff875e 100644
--- a/template/src/aws_central_infrastructure/iac_management/lib/github_oidc_lib.py
+++ b/template/src/aws_central_infrastructure/iac_management/lib/github_oidc_lib.py
@@ -1,4 +1,6 @@
+from typing import Self
 from typing import TypedDict
+from typing import TypeGuard
 
 from ephemeral_pulumi_deploy import get_config_str
 from ephemeral_pulumi_deploy.utils import common_tags_native
@@ -20,8 +22,14 @@
 from pydantic import BaseModel
 from pydantic import ConfigDict
 from pydantic import Field
+from pydantic import field_validator
+from pydantic import model_validator
+
+from .constants import GITHUB_ORG_IDS
 
 GITHUB_OIDC_URL = "https://token.actions.githubusercontent.com"
+IAM_STRING_LIKE_WILDCARD_CHARS = "*?"
+ANY_SUBJECT_CONTEXT = "*"
 CODE_ARTIFACT_SERVICE_BEARER_STATEMENT = GetPolicyDocumentStatementArgs(
     sid="GetCodeArtifactAuthToken",
     effect="Allow",
@@ -81,6 +89,14 @@ def _none_if_empty[T](items: list[T]) -> list[T] | None:
     return items
 
 
+def _is_specific_restriction(restriction: str | None) -> TypeGuard[str]:
+    if restriction is None:
+        return False
+    if restriction == ANY_SUBJECT_CONTEXT:
+        return False
+    return True
+
+
 class GithubOidcConfig(BaseModel):
     aws_account_id: str
     role_name: str
@@ -93,6 +109,36 @@ class GithubOidcConfig(BaseModel):
 
     model_config = ConfigDict(arbitrary_types_allowed=True)
 
+    @field_validator("repo_org")
+    @classmethod
+    def _require_known_org_id(cls, value: str) -> str:
+        if value not in GITHUB_ORG_IDS:
+            raise ValueError(  # noqa: TRY003 # pydantic validators must raise ValueError for it to be converted into a ValidationError
+                f'GitHub org {value!r} has no entry in GITHUB_ORG_IDS; add it (find the ID in the "id" field at https://api.github.com/orgs/{value} or with `gh api orgs/{value} --jq .id`)'
+            )
+        return value
+
+    @field_validator("restrictions")
+    @classmethod
+    def _allow_only_bare_wildcard(cls, value: str | None) -> str | None:
+        if not _is_specific_restriction(value):
+            return value
+        for wildcard_char in IAM_STRING_LIKE_WILDCARD_CHARS:
+            if wildcard_char in value:
+                raise ValueError(f"OIDC restriction {value!r} must not contain wildcard characters")  # noqa: TRY003 # pydantic validators must raise ValueError for it to be converted into a ValidationError
+        return value
+
+    @model_validator(mode="after")
+    def _reject_wildcard_repo_name_when_restricted(self) -> Self:
+        if not _is_specific_restriction(self.restrictions):
+            return self
+        for wildcard_char in IAM_STRING_LIKE_WILDCARD_CHARS:
+            if wildcard_char in self.repo_name:
+                raise ValueError(  # noqa: TRY003 # pydantic validators must raise ValueError for it to be converted into a ValidationError
+                    f"OIDC repo name {self.repo_name!r} must not contain wildcard characters when restricted to {self.restrictions!r}"
+                )
+        return self
+
     def create_role(self, *, provider_arn: str, parent: Resource | None = None) -> iam.Role:
         return iam.Role(
             f"{self.role_resource_name_prefix}{self.role_name}",
@@ -110,6 +156,17 @@ def create_role(self, *, provider_arn: str, parent: Resource | None = None) -> i
 def create_oidc_assume_role_policy(
     *, oidc_config: GithubOidcConfig, provider_arn: str
 ) -> AwaitableGetPolicyDocumentResult:
+    if oidc_config.restrictions is None:
+        subject_context = ANY_SUBJECT_CONTEXT
+    else:
+        subject_context = oidc_config.restrictions
+    # TODO: remove the legacy subject format once use_immutable_subject is enabled for every repo in every org in GITHUB_ORG_IDS. GitHub has announced no retirement date; existing repos keep the legacy format until opted in. Check that nothing else still trusts only the legacy format before opting in.
+    legacy_subject = f"repo:{oidc_config.repo_org}/{oidc_config.repo_name}:{subject_context}"
+    # Immutable subject format, the default for repos created after 2026-07-15: https://github.blog/changelog/2026-04-23-immutable-subject-claims-for-github-actions-oidc-tokens/
+    # TODO: pin the exact repo ID instead of the `@*` wildcard, sourced from the github-repos stack outputs (requires iac-management to run after github-repos) or a GitHub API lookup.
+    # The wildcard is acceptable for now: the org ID is pinned, so a renamed or squatted org cannot match; the literal `@` after the repo name stops similarly prefixed repo names from matching;
+    # the remaining exposure is a repo in this org being deleted and recreated under the same name, which requires an org admin, who can already edit these roles; and the legacy format kept alongside is weaker anyway.
+    immutable_subject = f"repo:{oidc_config.repo_org}@{GITHUB_ORG_IDS[oidc_config.repo_org]}/{oidc_config.repo_name}@*:{subject_context}"
     return get_policy_document(
         statements=[
             GetPolicyDocumentStatementArgs(
@@ -118,13 +175,9 @@ def create_oidc_assume_role_policy(
                 actions=["sts:AssumeRoleWithWebIdentity"],
                 conditions=[
                     GetPolicyDocumentStatementConditionArgs(
-                        test="StringLike"
-                        if oidc_config.restrictions is None or oidc_config.restrictions == "*"
-                        else "StringEquals",
+                        test="StringLike",
                         variable="token.actions.githubusercontent.com:sub",
-                        values=[
-                            f"repo:{oidc_config.repo_org}/{oidc_config.repo_name}:{'*' if oidc_config.restrictions is None else oidc_config.restrictions}"
-                        ],
+                        values=[legacy_subject, immutable_subject],
                     ),
                     GetPolicyDocumentStatementConditionArgs(
                         test="StringEquals",
diff --git a/template/src/aws_central_infrastructure/{% if manage_github_repos %}github_repos{% endif %}/lib/repo.py b/template/src/aws_central_infrastructure/{% if manage_github_repos %}github_repos{% endif %}/lib/repo.py
index ccf350b5..88aed9da 100644
--- a/template/src/aws_central_infrastructure/{% if manage_github_repos %}github_repos{% endif %}/lib/repo.py	
+++ b/template/src/aws_central_infrastructure/{% if manage_github_repos %}github_repos{% endif %}/lib/repo.py	
@@ -130,6 +130,7 @@ def __init__(
             repo_topics = ["managed-by-aws-central-infrastructure-iac-repo"]
             repo_topics += config.topics
 
+            # TODO: enable GitHub's immutable OIDC subject claims for every repo (ActionsRepositoryOidcSubjectClaimCustomizationTemplate with use_immutable_subject) once the provider supports it: https://github.com/integrations/terraform-provider-github/pull/3582 . That unblocks removing the legacy subject format from the trust policy in create_oidc_assume_role_policy.
             repo = Repository(
                 append_resource_suffix(config.name, max_length=150),
                 name=config.name,
diff --git a/template/tests/unit/iac_management/__init__.py b/template/tests/unit/iac_management/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/template/tests/unit/iac_management/test_oidc_assume_role_policy.py b/template/tests/unit/iac_management/test_oidc_assume_role_policy.py
new file mode 100644
index 00000000..17e3448d
--- /dev/null
+++ b/template/tests/unit/iac_management/test_oidc_assume_role_policy.py
@@ -0,0 +1,176 @@
+import random
+import re
+import string
+from unittest.mock import ANY
+from unittest.mock import MagicMock
+
+import pydantic
+import pytest
+from pulumi_aws.iam import GetPolicyDocumentStatementConditionArgs
+from pulumi_aws.iam import get_policy_document
+from pytest_mock import MockerFixture
+
+import aws_central_infrastructure.iac_management.lib.github_oidc_lib as github_oidc_lib_module
+from aws_central_infrastructure.iac_management.lib.constants import CENTRAL_INFRA_GITHUB_ORG_NAME
+from aws_central_infrastructure.iac_management.lib.constants import GITHUB_ORG_IDS
+from aws_central_infrastructure.iac_management.lib.github_oidc_lib import ANY_SUBJECT_CONTEXT
+from aws_central_infrastructure.iac_management.lib.github_oidc_lib import GithubOidcConfig
+from aws_central_infrastructure.iac_management.lib.github_oidc_lib import create_oidc_assume_role_policy
+
+CENTRAL_ORG_IMMUTABLE_PREFIX = f"{CENTRAL_INFRA_GITHUB_ORG_NAME}@{GITHUB_ORG_IDS[CENTRAL_INFRA_GITHUB_ORG_NAME]}"
+
+
+def _random_account_id() -> str:
+    return "".join(random.choices(string.digits, k=12))
+
+
+def _random_name() -> str:
+    return "".join(random.choices(string.ascii_lowercase + "-", k=8))
+
+
+def _sub_conditions_passed_to(mock_get_policy_document: MagicMock) -> list[GetPolicyDocumentStatementConditionArgs]:
+    (statement,) = mock_get_policy_document.call_args.kwargs["statements"]
+    return [
+        condition
+        for condition in statement.conditions
+        if condition.variable == "token.actions.githubusercontent.com:sub"
+    ]
+
+
+class TestCreateOidcAssumeRolePolicy:
+    def test_Given_no_restrictions__When_policy_created__Then_sub_matches_legacy_and_immutable_subjects(
+        self, mocker: MockerFixture
+    ) -> None:
+        repo_name = _random_name()
+        oidc_config = GithubOidcConfig(
+            aws_account_id=_random_account_id(),
+            role_name=_random_name(),
+            repo_org=CENTRAL_INFRA_GITHUB_ORG_NAME,
+            repo_name=repo_name,
+        )
+        mock_get_policy_document = mocker.patch.object(github_oidc_lib_module, get_policy_document.__name__)
+
+        _ = create_oidc_assume_role_policy(oidc_config=oidc_config, provider_arn=_random_name())
+
+        sub_conditions = _sub_conditions_passed_to(mock_get_policy_document)
+
+        mock_get_policy_document.assert_called_once_with(statements=[ANY])
+        assert len(sub_conditions) == 1
+        assert sub_conditions[0].test == "StringLike"
+        assert sub_conditions[0].values == [
+            f"repo:{CENTRAL_INFRA_GITHUB_ORG_NAME}/{repo_name}:*",
+            f"repo:{CENTRAL_ORG_IMMUTABLE_PREFIX}/{repo_name}@*:*",
+        ]
+
+    def test_Given_ref_restriction__When_policy_created__Then_sub_matches_both_subjects_scoped_to_that_ref(
+        self, mocker: MockerFixture
+    ) -> None:
+        repo_name = _random_name()
+        restriction = f"ref:refs/heads/{_random_name()}"
+        oidc_config = GithubOidcConfig(
+            aws_account_id=_random_account_id(),
+            role_name=_random_name(),
+            repo_org=CENTRAL_INFRA_GITHUB_ORG_NAME,
+            repo_name=repo_name,
+            restrictions=restriction,
+        )
+        mock_get_policy_document = mocker.patch.object(github_oidc_lib_module, get_policy_document.__name__)
+
+        _ = create_oidc_assume_role_policy(oidc_config=oidc_config, provider_arn=_random_name())
+
+        sub_conditions = _sub_conditions_passed_to(mock_get_policy_document)
+
+        mock_get_policy_document.assert_called_once_with(statements=[ANY])
+        assert len(sub_conditions) == 1
+        assert sub_conditions[0].test == "StringLike"
+        assert sub_conditions[0].values == [
+            f"repo:{CENTRAL_INFRA_GITHUB_ORG_NAME}/{repo_name}:{restriction}",
+            f"repo:{CENTRAL_ORG_IMMUTABLE_PREFIX}/{repo_name}@*:{restriction}",
+        ]
+
+
+class TestWhenGithubOidcConfigCreated:
+    @pytest.mark.parametrize("wildcard_char", ["*", "?"])
+    def test_Given_restriction_containing_wildcard__Then_validation_error_names_restriction(
+        self, wildcard_char: str
+    ) -> None:
+        restriction = f"ref:refs/heads/{_random_name()}{wildcard_char}"
+
+        with pytest.raises(pydantic.ValidationError, match=re.escape(restriction)):
+            _ = GithubOidcConfig(
+                aws_account_id=_random_account_id(),
+                role_name=_random_name(),
+                repo_org=CENTRAL_INFRA_GITHUB_ORG_NAME,
+                repo_name=_random_name(),
+                restrictions=restriction,
+            )
+
+    def test_Given_bare_wildcard_restriction__Then_config_keeps_it(self) -> None:
+        oidc_config = GithubOidcConfig(
+            aws_account_id=_random_account_id(),
+            role_name=_random_name(),
+            repo_org=CENTRAL_INFRA_GITHUB_ORG_NAME,
+            repo_name=_random_name(),
+            restrictions=ANY_SUBJECT_CONTEXT,
+        )
+
+        assert oidc_config.restrictions == ANY_SUBJECT_CONTEXT
+
+    def test_Given_restrictions_explicitly_none__Then_config_has_no_restrictions(self) -> None:
+        oidc_config = GithubOidcConfig(
+            aws_account_id=_random_account_id(),
+            role_name=_random_name(),
+            repo_org=CENTRAL_INFRA_GITHUB_ORG_NAME,
+            repo_name=_random_name(),
+            restrictions=None,
+        )
+
+        assert oidc_config.restrictions is None
+
+    @pytest.mark.parametrize("wildcard_char", ["*", "?"])
+    def test_Given_wildcard_repo_name_with_ref_restriction__Then_validation_error_names_repo_and_restriction(
+        self, wildcard_char: str
+    ) -> None:
+        repo_name = f"{_random_name()}{wildcard_char}"
+        restriction = f"ref:refs/heads/{_random_name()}"
+
+        with pytest.raises(pydantic.ValidationError, match=f"{re.escape(repo_name)}.*{re.escape(restriction)}"):
+            _ = GithubOidcConfig(
+                aws_account_id=_random_account_id(),
+                role_name=_random_name(),
+                repo_org=CENTRAL_INFRA_GITHUB_ORG_NAME,
+                repo_name=repo_name,
+                restrictions=restriction,
+            )
+
+    def test_Given_wildcard_repo_name_without_restriction__Then_config_keeps_it(self) -> None:
+        oidc_config = GithubOidcConfig(
+            aws_account_id=_random_account_id(),
+            role_name=_random_name(),
+            repo_org=CENTRAL_INFRA_GITHUB_ORG_NAME,
+            repo_name="*",
+        )
+
+        assert oidc_config.repo_name == "*"
+
+    def test_Given_wildcard_repo_name_with_bare_wildcard_restriction__Then_config_keeps_it(self) -> None:
+        oidc_config = GithubOidcConfig(
+            aws_account_id=_random_account_id(),
+            role_name=_random_name(),
+            repo_org=CENTRAL_INFRA_GITHUB_ORG_NAME,
+            repo_name="*",
+            restrictions=ANY_SUBJECT_CONTEXT,
+        )
+
+        assert oidc_config.repo_name == "*"
+
+    def test_Given_repo_org_without_known_org_id__Then_validation_error_names_org(self) -> None:
+        unknown_org_name = _random_name()
+
+        with pytest.raises(pydantic.ValidationError, match=re.escape(unknown_org_name)):
+            _ = GithubOidcConfig(
+                aws_account_id=_random_account_id(),
+                role_name=_random_name(),
+                repo_org=unknown_org_name,
+                repo_name=_random_name(),
+            )
diff --git a/tests/copier_data/data1.yaml b/tests/copier_data/data1.yaml
index e8168e97..726f2c50 100644
--- a/tests/copier_data/data1.yaml
+++ b/tests/copier_data/data1.yaml
@@ -29,6 +29,7 @@ aws_region_for_stack: us-east-2
 
 # Data added based on the specifics of this template
 central_infra_github_organization_name: my-org
+central_infra_github_organization_id: 7
 initial_iac_management_deploy_occurred: false
 identity_center_production_account_id: 023202320444
 configure_cloud_courier: false
diff --git a/tests/copier_data/data2.yaml b/tests/copier_data/data2.yaml
index 0d651fb6..1c28e4dc 100644
--- a/tests/copier_data/data2.yaml
+++ b/tests/copier_data/data2.yaml
@@ -34,6 +34,7 @@ aws_region_for_stack: us-west-1
 
 # Data added based on the specifics of this template
 central_infra_github_organization_name: foobar
+central_infra_github_organization_id: 987654321
 initial_iac_management_deploy_occurred: true
 identity_center_production_account_id: 123499989012
 configure_cloud_courier: true
diff --git a/tests/copier_data/data3.yaml b/tests/copier_data/data3.yaml
index b9f27f98..c85b0e63 100644
--- a/tests/copier_data/data3.yaml
+++ b/tests/copier_data/data3.yaml
@@ -24,6 +24,7 @@ aws_development_account_id: 000321456870
 aws_region_for_stack: us-west-1
 # Data added based on the specifics of this template
 central_infra_github_organization_name: foobar
+central_infra_github_organization_id: 4294967296
 initial_iac_management_deploy_occurred: true
 identity_center_production_account_id: 123499989012
 configure_cloud_courier: true