diff --git a/server/channels/api4/team.go b/server/channels/api4/team.go
index f8b6ec6f10c..021c3c6dab6 100644
--- a/server/channels/api4/team.go
+++ b/server/channels/api4/team.go
@@ -1375,7 +1375,7 @@ func updateTeamMemberRoles(c *Context, w http.ResponseWriter, r *http.Request) {
 	props := model.MapFromJSON(r.Body)
 
 	newRoles := props["roles"]
-	if !model.IsValidUserRoles(newRoles) {
+	if !model.IsValidTeamMemberRoles(newRoles) {
 		c.SetInvalidParam("team_member_roles")
 		return
 	}
diff --git a/server/channels/api4/team_test.go b/server/channels/api4/team_test.go
index 735b4b86d31..10610eaaecb 100644
--- a/server/channels/api4/team_test.go
+++ b/server/channels/api4/team_test.go
@@ -3872,6 +3872,63 @@ func TestUpdateTeamMemberRolesRejectsGuestAndAdmin(t *testing.T) {
 	require.Equal(t, rolesBefore, memberAfter.Roles)
 }
 
+func TestUpdateTeamMemberRolesRejectsNonTeamScopedRoles(t *testing.T) {
+	mainHelper.Parallel(t)
+	th := Setup(t).InitBasic(t)
+	SystemAdminClient := th.SystemAdminClient
+
+	const teamMember = "team_user"
+
+	invalidRoles := []struct {
+		name  string
+		roles string
+	}{
+		{name: "system manager with team user", roles: teamMember + " " + model.SystemManagerRoleId},
+		{name: "system user manager with team user", roles: teamMember + " " + model.SystemUserManagerRoleId},
+		{name: "system read only admin with team user", roles: teamMember + " " + model.SystemReadOnlyAdminRoleId},
+		{name: "system post all with team user", roles: teamMember + " " + model.SystemPostAllRoleId},
+		{name: "team post all with team user", roles: teamMember + " " + model.TeamPostAllRoleId},
+		{name: "team post all public with team user", roles: teamMember + " " + model.TeamPostAllPublicRoleId},
+		{name: "channel user with team user", roles: teamMember + " " + model.ChannelUserRoleId},
+		{name: "custom group user with team user", roles: teamMember + " " + model.CustomGroupUserRoleId},
+	}
+
+	for _, tc := range invalidRoles {
+		t.Run("rejects "+tc.name, func(t *testing.T) {
+			memberBefore, _, err := SystemAdminClient.GetTeamMember(context.Background(), th.BasicTeam.Id, th.BasicUser2.Id, "")
+			require.NoError(t, err)
+			rolesBefore := memberBefore.Roles
+
+			resp, err := SystemAdminClient.UpdateTeamMemberRoles(context.Background(), th.BasicTeam.Id, th.BasicUser2.Id, tc.roles)
+			require.Error(t, err)
+			CheckBadRequestStatus(t, resp)
+
+			memberAfter, _, err := SystemAdminClient.GetTeamMember(context.Background(), th.BasicTeam.Id, th.BasicUser2.Id, "")
+			require.NoError(t, err)
+			require.Equal(t, rolesBefore, memberAfter.Roles)
+		})
+	}
+
+	validRoles := []struct {
+		name  string
+		roles string
+	}{
+		{name: "team member", roles: teamMember},
+		{name: "team admin", roles: "team_user team_admin"},
+	}
+
+	for _, tc := range validRoles {
+		t.Run("accepts "+tc.name, func(t *testing.T) {
+			_, err := SystemAdminClient.UpdateTeamMemberRoles(context.Background(), th.BasicTeam.Id, th.BasicUser2.Id, tc.roles)
+			require.NoError(t, err)
+
+			member, _, err := SystemAdminClient.GetTeamMember(context.Background(), th.BasicTeam.Id, th.BasicUser2.Id, "")
+			require.NoError(t, err)
+			require.ElementsMatch(t, strings.Fields(tc.roles), strings.Fields(member.Roles))
+		})
+	}
+}
+
 func TestUpdateTeamMemberSchemeRoles(t *testing.T) {
 	mainHelper.Parallel(t)
 	th := Setup(t).InitBasic(t)
diff --git a/server/public/model/role.go b/server/public/model/role.go
index e0327e8fac3..8380d19a6a5 100644
--- a/server/public/model/role.go
+++ b/server/public/model/role.go
@@ -5,6 +5,7 @@ package model
 
 import (
 	"fmt"
+	"slices"
 	"strings"
 
 	"github.com/mattermost/mattermost/server/public/utils/timeutils"
@@ -893,22 +894,38 @@ func IsBuiltInRole(roleName string) bool {
 	return builtInRoleSet[roleName]
 }
 
+// channelScopedBuiltInRoles are the built-in roles valid inside a channel-member role list.
+var channelScopedBuiltInRoles = []string{ChannelGuestRoleId, ChannelUserRoleId, ChannelAdminRoleId}
+
+// teamScopedBuiltInRoles are the built-in roles valid inside a team-member role list.
+var teamScopedBuiltInRoles = []string{TeamGuestRoleId, TeamUserRoleId, TeamAdminRoleId}
+
 // IsChannelScopedBuiltInRole returns true for the three built-in roles that are
 // valid inside a channel-member role list.
 func IsChannelScopedBuiltInRole(roleName string) bool {
-	return roleName == ChannelGuestRoleId || roleName == ChannelUserRoleId || roleName == ChannelAdminRoleId
+	return slices.Contains(channelScopedBuiltInRoles, roleName)
 }
 
 // IsValidChannelMemberRoles reports whether roles are valid for a channel member.
-// IsValidUserRoles is format validation only; this additionally rejects any built-in
-// role (per IsBuiltInRole) that is not channel-scoped.
 func IsValidChannelMemberRoles(channelMemberRoles string) bool {
-	if !IsValidUserRoles(channelMemberRoles) {
+	return IsValidMemberRoles(channelMemberRoles, channelScopedBuiltInRoles)
+}
+
+// IsValidTeamMemberRoles reports whether roles are valid for a team member.
+func IsValidTeamMemberRoles(teamMemberRoles string) bool {
+	return IsValidMemberRoles(teamMemberRoles, teamScopedBuiltInRoles)
+}
+
+// IsValidMemberRoles reports whether memberRoles are valid for a member. IsValidUserRoles
+// is format validation only; this additionally rejects any built-in role (per IsBuiltInRole)
+// that is not in validRoles.
+func IsValidMemberRoles(memberRoles string, validRoles []string) bool {
+	if !IsValidUserRoles(memberRoles) {
 		return false
 	}
 
-	for roleName := range strings.FieldsSeq(channelMemberRoles) {
-		if IsBuiltInRole(roleName) && !IsChannelScopedBuiltInRole(roleName) {
+	for roleName := range strings.FieldsSeq(memberRoles) {
+		if IsBuiltInRole(roleName) && !slices.Contains(validRoles, roleName) {
 			return false
 		}
 	}
diff --git a/server/public/model/role_test.go b/server/public/model/role_test.go
index cb01b4a29a4..abd1ecbf1fc 100644
--- a/server/public/model/role_test.go
+++ b/server/public/model/role_test.go
@@ -566,6 +566,37 @@ func TestIsValidChannelMemberRoles(t *testing.T) {
 	}
 }
 
+func TestIsValidTeamMemberRoles(t *testing.T) {
+	tests := []struct {
+		name  string
+		roles string
+		valid bool
+	}{
+		{name: "team user only", roles: TeamUserRoleId, valid: true},
+		{name: "team user and admin", roles: TeamUserRoleId + " " + TeamAdminRoleId, valid: true},
+		{name: "team guest only", roles: TeamGuestRoleId, valid: true},
+		{name: "team post all with team user", roles: TeamUserRoleId + " " + TeamPostAllRoleId, valid: false},
+		{name: "team post all public with team user", roles: TeamUserRoleId + " " + TeamPostAllPublicRoleId, valid: false},
+		{name: "custom role with team user", roles: TeamUserRoleId + " custom_role", valid: true},
+		{name: "prefixed custom team role with team user", roles: TeamUserRoleId + " team_custom", valid: true},
+		{name: "prefixed custom system role with team user", roles: TeamUserRoleId + " system_custom", valid: true},
+		{name: "channel user with team user", roles: TeamUserRoleId + " " + ChannelUserRoleId, valid: false},
+		{name: "system user with team user", roles: TeamUserRoleId + " " + SystemUserRoleId, valid: false},
+		{name: "system manager with team user", roles: TeamUserRoleId + " " + SystemManagerRoleId, valid: false},
+		{name: "system post all with team user", roles: TeamUserRoleId + " " + SystemPostAllRoleId, valid: false},
+		{name: "system read only admin with team user", roles: TeamUserRoleId + " " + SystemReadOnlyAdminRoleId, valid: false},
+		{name: "custom group user with team user", roles: TeamUserRoleId + " " + CustomGroupUserRoleId, valid: false},
+		{name: "system custom group admin with team user", roles: TeamUserRoleId + " " + SystemCustomGroupAdminRoleId, valid: false},
+		{name: "invalid role name", roles: "invalid-role", valid: false},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			assert.Equal(t, tc.valid, IsValidTeamMemberRoles(tc.roles))
+		})
+	}
+}
+
 func TestIsBuiltInRole(t *testing.T) {
 	tests := []struct {
 		name     string