diff --git a/CHANGELOG.md b/CHANGELOG.md
index 837b027f..9a2ce082 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,9 @@
+0.22.29
+=======
+
+* Update expat to 2.9.0
+
+
 0.22.28
 =======
 
diff --git a/relenv/build/darwin.py b/relenv/build/darwin.py
index c35fb80f..228ac22f 100644
--- a/relenv/build/darwin.py
+++ b/relenv/build/darwin.py
@@ -119,6 +119,12 @@ def update_expat(dirs: Dirs, env: MutableMapping[str, str]) -> None:
             f.write('#if defined(HAVE_ARC4RANDOM_BUF)\n#include "random_arc4random_buf.c"\n#endif\n')
             f.write('#if defined(HAVE_ARC4RANDOM)\n#include "random_arc4random.c"\n#endif\n')
             f.write('#if !defined(_WIN32) && defined(XML_DEV_URANDOM)\n#include "random_dev_urandom.c"\n#endif\n')
+            # expat >= 2.9.0 moved xcslen/xcscmp/xcsncmp into a separate TU
+            # (xcs.c). Python's build system only compiles xmlparse.c,
+            # xmlrole.c, and xmltok.c, so pull xcs.c into xmlparse.c's TU to
+            # avoid undefined-reference failures when linking pyexpat.
+            if (expat_dir / "xcs.c").exists():
+                f.write('#include "xcs.c"\n')
 
     # Update SBOM with correct checksums for updated expat files
     files_to_update = {}
diff --git a/relenv/build/linux.py b/relenv/build/linux.py
index 62b10c9d..775b8326 100644
--- a/relenv/build/linux.py
+++ b/relenv/build/linux.py
@@ -476,6 +476,12 @@ def update_expat(dirs: Dirs, env: EnvMapping) -> None:
             f.write('#if defined(HAVE_ARC4RANDOM_BUF)\n#include "random_arc4random_buf.c"\n#endif\n')
             f.write('#if defined(HAVE_ARC4RANDOM)\n#include "random_arc4random.c"\n#endif\n')
             f.write('#if !defined(_WIN32) && defined(XML_DEV_URANDOM)\n#include "random_dev_urandom.c"\n#endif\n')
+            # expat >= 2.9.0 moved xcslen/xcscmp/xcsncmp into a separate TU
+            # (xcs.c). Python's build system only compiles xmlparse.c,
+            # xmlrole.c, and xmltok.c, so pull xcs.c into xmlparse.c's TU to
+            # avoid undefined-reference failures when linking pyexpat.
+            if (expat_dir / "xcs.c").exists():
+                f.write('#include "xcs.c"\n')
 
     # Update SBOM with correct checksums for updated expat files
     files_to_update = {}
diff --git a/relenv/build/windows.py b/relenv/build/windows.py
index 1a916b57..dd2b7ce6 100644
--- a/relenv/build/windows.py
+++ b/relenv/build/windows.py
@@ -446,6 +446,12 @@ def update_expat(dirs: Dirs, env: EnvMapping) -> None:
             f.write('#if defined(HAVE_ARC4RANDOM_BUF)\n#include "random_arc4random_buf.c"\n#endif\n')
             f.write('#if defined(HAVE_ARC4RANDOM)\n#include "random_arc4random.c"\n#endif\n')
             f.write('#if !defined(_WIN32) && defined(XML_DEV_URANDOM)\n#include "random_dev_urandom.c"\n#endif\n')
+            # expat >= 2.9.0 moved xcslen/xcscmp/xcsncmp into a separate TU
+            # (xcs.c). Python's build system only compiles xmlparse.c,
+            # xmlrole.c, and xmltok.c, so pull xcs.c into xmlparse.c's TU to
+            # avoid undefined-reference failures when linking pyexpat.
+            if (expat_dir / "xcs.c").exists():
+                f.write('#include "xcs.c"\n')
 
     # Update SBOM with correct checksums for updated expat files
     files_to_update = {f"Modules/expat/{f.name}": f for f in updated_files}
diff --git a/relenv/common.py b/relenv/common.py
index f1b0381e..5ca9d8b7 100644
--- a/relenv/common.py
+++ b/relenv/common.py
@@ -27,7 +27,7 @@
     from collections.abc import Callable, Iterable, Mapping
 
 # relenv package version
-__version__ = "0.22.28"
+__version__ = "0.22.29"
 
 log = logging.getLogger(__name__)
 
diff --git a/relenv/python-versions.json b/relenv/python-versions.json
index 774da6af..35ff9fe2 100644
--- a/relenv/python-versions.json
+++ b/relenv/python-versions.json
@@ -675,6 +675,15 @@
      "darwin",
      "win32"
     ]
+   },
+   "2.9.0": {
+    "url": "https://github.com/libexpat/libexpat/releases/download/R_2_9_0/expat-{version}.tar.xz",
+    "sha256": "1e6371862cc31999b368c3b89b49994f0677e1bab5f1b2b85ae3741f5d803051",
+    "platforms": [
+     "linux",
+     "darwin",
+     "win32"
+    ]
    }
   }
  }