diff --git a/docs/src/whatsnew/3.16/3.16.rst b/docs/src/whatsnew/3.16/3.16.rst
index 45cd3ed0a6..05a87597b5 100644
--- a/docs/src/whatsnew/3.16/3.16.rst
+++ b/docs/src/whatsnew/3.16/3.16.rst
@@ -160,6 +160,8 @@ v3.16 (13 Aug 2026)
 
 - :fa:`code-pull-request` :pull:`7223`: :user:`trexfeathers` updated Iris to be compatible with Matplotlib version 3.11.
 
+- :fa:`code-pull-request` :pull:`7262`: :user:`trexfeathers` updated Iris' metadata handling to be compatible with python-xxhash version 4.0 .
+
 
 
 📚 Documentation
diff --git a/lib/iris/common/metadata.py b/lib/iris/common/metadata.py
index c24f240217..cf8a8de1e8 100644
--- a/lib/iris/common/metadata.py
+++ b/lib/iris/common/metadata.py
@@ -96,6 +96,10 @@ def hexdigest(item):
         parts = (item.shape, xxh64_hexdigest(item))
         item = str(parts)
 
+    # xxhash 4.0+: str is no longer accepted; encode to bytes first.
+    if isinstance(item, str):
+        item = item.encode()
+
     try:
         # Calculate single-shot hash to avoid allocating state on the heap
         result = xxh64_hexdigest(item)
@@ -104,7 +108,7 @@ def hexdigest(item):
         # string representation of the provided item instead, but
         # also fold in the object type...
         parts = (type(item), item)
-        result = xxh64_hexdigest(str(parts))
+        result = xxh64_hexdigest(str(parts).encode())
 
     return result
 
diff --git a/lib/iris/tests/unit/common/metadata/test_hexdigest.py b/lib/iris/tests/unit/common/metadata/test_hexdigest.py
index 0ce0dcd706..05285dc34f 100644
--- a/lib/iris/tests/unit/common/metadata/test_hexdigest.py
+++ b/lib/iris/tests/unit/common/metadata/test_hexdigest.py
@@ -21,7 +21,7 @@ def _setup(self):
     @staticmethod
     def _ndarray(value):
         parts = str((value.shape, xxh64_hexdigest(value)))
-        return xxh64_hexdigest(parts)
+        return xxh64_hexdigest(parts.encode())
 
     @staticmethod
     def _masked(value):
@@ -32,11 +32,11 @@ def _masked(value):
                 xxh64_hexdigest(value.mask),
             )
         )
-        return xxh64_hexdigest(parts)
+        return xxh64_hexdigest(parts.encode())
 
     def test_string(self):
         value = "hello world"
-        self.hasher.update(value)
+        self.hasher.update(value.encode())
         expected = self.hasher.hexdigest()
         assert hexdigest(value) == expected
 
@@ -111,7 +111,7 @@ def test_masked_array_reshape_not_flat(self):
 class TestNotBytesLikeObject:
     def _expected(self, value):
         parts = str((type(value), value))
-        return xxh64_hexdigest(parts)
+        return xxh64_hexdigest(parts.encode())
 
     def test_int(self):
         value = 123