diff --git a/eigen/build.sh b/eigen/build.sh
new file mode 100755
index 0000000..6a55892
--- /dev/null
+++ b/eigen/build.sh
@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+set -e
+
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null && pwd)"
+cd "$DIR"
+
+VERSION="3.4.0"
+INSTALL_DIR="$DIR/eigen/install"
+
+# Idempotent: skip if already present
+if [ -d "$INSTALL_DIR/eigen3/Eigen" ]; then
+  echo "eigen already present, skipping download."
+  exit 0
+fi
+
+TARBALL="eigen-${VERSION}.tar.gz"
+URL="https://gitlab.com/libeigen/eigen/-/archive/${VERSION}/${TARBALL}"
+
+echo "Downloading Eigen ${VERSION} ..."
+curl -fSL -o "$TARBALL" "$URL"
+
+echo "Extracting headers ..."
+rm -rf "$INSTALL_DIR"
+mkdir -p "$INSTALL_DIR/eigen3"
+
+# Extract only the Eigen/ and unsupported/Eigen/ header directories
+tar --strip-components=1 -xzf "$TARBALL" -C "$INSTALL_DIR/eigen3" \
+  "eigen-${VERSION}/Eigen" \
+  "eigen-${VERSION}/unsupported"
+
+rm -f "$TARBALL"
+
+echo "Installed eigen to $INSTALL_DIR"
+du -sh "$INSTALL_DIR"
diff --git a/eigen/eigen/__init__.py b/eigen/eigen/__init__.py
new file mode 100644
index 0000000..5b1151e
--- /dev/null
+++ b/eigen/eigen/__init__.py
@@ -0,0 +1,9 @@
+import os
+
+DIR = os.path.join(os.path.dirname(__file__), "install")
+INCLUDE_DIR = DIR
+LIB_DIR = DIR  # header-only; no libraries
+
+
+def smoketest():
+  assert os.path.isdir(os.path.join(DIR, "eigen3", "Eigen")), "Eigen headers not found"
diff --git a/eigen/pyproject.toml b/eigen/pyproject.toml
new file mode 100644
index 0000000..8928294
--- /dev/null
+++ b/eigen/pyproject.toml
@@ -0,0 +1,15 @@
+[build-system]
+requires = ["setuptools>=64", "wheel"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "eigen"
+version = "3.4.0"
+description = "Eigen C++ linear algebra headers"
+requires-python = ">=3.8"
+
+[tool.setuptools.packages.find]
+include = ["eigen*"]
+
+[tool.setuptools.package-data]
+eigen = ["install/**/*"]
diff --git a/eigen/setup.py b/eigen/setup.py
new file mode 100644
index 0000000..de858b5
--- /dev/null
+++ b/eigen/setup.py
@@ -0,0 +1,61 @@
+import os
+import platform
+import subprocess
+
+from setuptools.command.build_py import build_py
+
+try:
+  from wheel.bdist_wheel import bdist_wheel
+except ImportError:
+  bdist_wheel = None
+
+
+class BuildEigen(build_py):
+  """Run build.sh to download Eigen headers before collecting package data."""
+
+  def run(self):
+    pkg_dir = os.path.dirname(os.path.abspath(__file__))
+    marker = os.path.join(pkg_dir, "eigen", "install", "eigen3", "Eigen")
+
+    if not os.path.isdir(marker):
+      build_script = os.path.join(pkg_dir, "build.sh")
+      subprocess.check_call(["bash", build_script], cwd=pkg_dir)
+
+    super().run()
+
+
+cmdclass = {"build_py": BuildEigen}
+
+if bdist_wheel is not None:
+
+  class PlatformWheel(bdist_wheel):
+    """Produce a platform-specific, Python-version-agnostic wheel."""
+
+    def finalize_options(self):
+      super().finalize_options()
+      self.root_is_pure = False
+
+    def get_tag(self):
+      system = platform.system()
+      machine = platform.machine()
+
+      if system == "Linux":
+        plat = f"linux_{machine}"
+      elif system == "Darwin":
+        plat = "macosx_11_0_arm64"
+      else:
+        plat = f"{system.lower()}_{machine}"
+
+      return "py3", "none", plat
+
+  cmdclass["bdist_wheel"] = PlatformWheel
+
+
+def setup():
+  from setuptools import setup as _setup
+
+  _setup(cmdclass=cmdclass)
+
+
+if __name__ == "__main__":
+  setup()