diff --git a/iodata/formats/trexio.py b/iodata/formats/trexio.py
new file mode 100644
index 000000000..ee41bf070
--- /dev/null
+++ b/iodata/formats/trexio.py
@@ -0,0 +1,162 @@
+# IODATA is an input and output module for quantum chemistry.
+# Copyright (C) 2011-2019 The IODATA Development Team
+#
+# This file is part of IODATA.
+#
+# IODATA is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 3
+# of the License, or (at your option) any later version.
+#
+# IODATA is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, see <http://www.gnu.org/licenses/>
+# --
+"""TrexIO file format."""
+
+from __future__ import annotations
+
+import os
+from typing import TextIO
+
+import numpy as np
+
+from ..docstrings import document_dump_one, document_load_one
+from ..iodata import IOData
+from ..utils import LineIterator, LoadError
+
+__all__ = ()
+
+PATTERNS = ["*.trexio"]
+
+
+def _import_trexio():
+    """Lazily import the trexio module."""
+    try:
+        import trexio  # noqa: PLC0415
+    except ImportError:
+        return None
+    return trexio
+
+
+@document_load_one(
+    "TREXIO",
+    ["atcoords", "atnums"],
+    ["charge", "nelec", "spinpol"],
+)
+def load_one(lit: LineIterator) -> dict:
+    """Do not edit this docstring. It will be overwritten."""
+    trexio = _import_trexio()
+    filename = lit.filename
+
+    if trexio is None:
+        raise LoadError(
+            "Reading TREXIO files requires the 'trexio' Python package.",
+            filename,
+        )
+
+    try:
+        # TrexIO needs to open the file itself.
+        # We cannot close lit.f because LineIterator might use it or context manager needs it.
+        # On Unix, opening same file twice for read is usually OK.
+        with trexio.File(filename, "r", back_end=trexio.TREXIO_HDF5) as tfile:
+            n_nuc = trexio.read_nucleus_num(tfile)
+            charges = np.asarray(trexio.read_nucleus_charge(tfile), dtype=float)
+            coords = np.asarray(trexio.read_nucleus_coord(tfile), dtype=float)
+
+            try:
+                nelec = int(trexio.read_electron_num(tfile))
+            except trexio.Error:
+                nelec = None
+
+            try:
+                n_up = int(trexio.read_electron_up_num(tfile))
+                n_dn = int(trexio.read_electron_dn_num(tfile))
+                spinpol = n_up - n_dn
+            except trexio.Error:
+                spinpol = None
+
+    except LoadError:
+        raise
+    except Exception as exc:
+        raise LoadError(f"Failed to read TREXIO file: {exc}", filename) from exc
+
+    # Validate data consistency after reading
+    if charges.shape[0] != n_nuc or coords.shape[0] != n_nuc:
+        raise LoadError(
+            "Inconsistent nucleus.* fields in TREXIO file.",
+            filename,
+        )
+
+    atnums = np.rint(charges).astype(int)
+
+    result: dict = {
+        "atcoords": coords,
+        "atnums": atnums,
+    }
+
+    if nelec is not None:
+        result["nelec"] = nelec
+        result["charge"] = float(charges.sum() - nelec)
+    if spinpol is not None:
+        result["spinpol"] = spinpol
+
+    return result
+
+
+@document_dump_one(
+    "TREXIO",
+    ["atcoords", "atnums"],
+    ["charge", "nelec", "spinpol"],
+)
+def dump_one(f: TextIO, data: IOData):
+    """Do not edit this docstring. It will be overwritten."""
+    trexio = _import_trexio()
+    if trexio is None:
+        raise RuntimeError("Writing TREXIO files requires the 'trexio' Python package.")
+
+    if data.atcoords is None or data.atnums is None:
+        raise RuntimeError("TREXIO writer needs atcoords and atnums.")
+    if data.atcoords.shape[0] != data.atnums.shape[0]:
+        raise RuntimeError("Inconsistent number of atoms in atcoords and atnums.")
+
+    try:
+        filename = f.name
+    except AttributeError as exc:
+        raise RuntimeError(
+            "TREXIO writer expects a real file object with a .name attribute."
+        ) from exc
+
+    atcoords = np.asarray(data.atcoords, dtype=float)
+    atnums = np.asarray(data.atnums, dtype=float)
+    nelec = int(data.nelec) if data.nelec is not None else None
+    spinpol = int(data.spinpol) if data.spinpol is not None else None
+
+    # TrexIO needs to open the file itself. We close the file handle provided by api.py
+    # to avoid conflicts (e.g. file locking). api.py will harmlessly close it again.
+    f.close()
+    if os.path.exists(filename):
+        os.remove(filename)
+
+    with trexio.File(filename, "w", back_end=trexio.TREXIO_HDF5) as tfile:
+        trexio.write_nucleus_num(tfile, len(atnums))
+        trexio.write_nucleus_charge(tfile, atnums.astype(float))
+        trexio.write_nucleus_coord(tfile, atcoords)
+
+        if nelec is not None:
+            trexio.write_electron_num(tfile, nelec)
+            if spinpol is not None:
+                # Check for consistency between nelec and spinpol
+                if abs((nelec + spinpol) % 2) > 1.0e-8:
+                    raise ValueError(
+                        f"Inconsistent nelec ({nelec}) and spinpol ({spinpol}). "
+                        "Sum and difference must be even numbers."
+                    )
+                n_up = (nelec + spinpol) // 2
+                n_dn = (nelec - spinpol) // 2
+                trexio.write_electron_up_num(tfile, n_up)
+                trexio.write_electron_dn_num(tfile, n_dn)
diff --git a/iodata/test/test_trexio.py b/iodata/test/test_trexio.py
new file mode 100644
index 000000000..7058c7ace
--- /dev/null
+++ b/iodata/test/test_trexio.py
@@ -0,0 +1,96 @@
+# IODATA is an input and output module for quantum chemistry.
+# Copyright (C) 2011-2019 The IODATA Development Team
+#
+# This file is part of IODATA.
+#
+# IODATA is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 3
+# of the License, or (at your option) any later version.
+#
+# IODATA is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, see <http://www.gnu.org/licenses/>
+# --
+import importlib.util
+import os
+import subprocess
+import sys
+
+import pytest
+
+
+@pytest.mark.skipif(sys.platform.startswith("win"), reason="TrexIO issues on Windows")
+def test_load_dump_consistency(tmp_path):
+    """Check if dumping and loading a TREXIO file results in the same data.
+
+    Runs in a subprocess to avoid segmentation faults caused by conflict
+    between pytest execution model and trexio C-extension.
+    """
+    # Skip tests if trexio is not installed, but do NOT import it here to avoid segfaults
+    if importlib.util.find_spec("trexio") is None:
+        pytest.skip("trexio not installed")
+    script = """
+import numpy as np
+import os
+import sys
+
+from iodata import IOData
+from iodata.api import load_one, dump_one
+
+atcoords = np.array([[0.0, 0.0, 0.0], [1.0, 0.0, 0.0]])
+atnums = np.array([1, 1])
+nelec = 2.0
+spinpol = 0
+iodata_orig = IOData(atcoords=atcoords, atnums=atnums, nelec=nelec, spinpol=spinpol)
+
+filename = "test.trexio"
+if os.path.exists(filename):
+    os.remove(filename)
+
+print(f"Dumping to {filename}")
+dump_one(iodata_orig, filename, fmt="trexio")
+
+print(f"Loading from {filename}")
+iodata_new = load_one(filename, fmt="trexio")
+
+print("Verifying data...")
+np.testing.assert_allclose(iodata_new.atcoords, atcoords, err_msg="atcoords mismatch")
+np.testing.assert_equal(iodata_new.atnums, atnums, err_msg="atnums mismatch")
+np.testing.assert_allclose(
+    float(iodata_new.nelec),
+    nelec,
+    rtol=1.0e-8,
+    atol=1.0e-12,
+    err_msg=f"nelec mismatch: {iodata_new.nelec} != {nelec}",
+)
+np.testing.assert_allclose(
+    float(iodata_new.charge),
+    0.0,
+    rtol=1.0e-8,
+    atol=1.0e-12,
+    err_msg=f"charge mismatch: {iodata_new.charge} != 0.0",
+)
+assert int(iodata_new.spinpol) == spinpol, (
+    f"spinpol mismatch: {iodata_new.spinpol} != {spinpol}"
+)
+
+print("Verification passed")
+"""
+    script_file = tmp_path / "verify_trexio_subprocess.py"
+    script_file.write_text(script, encoding="utf-8")
+
+    # Determine project root (assuming this test is in iodata/test/)
+    current_dir = os.path.dirname(os.path.abspath(__file__))
+    project_root = os.path.abspath(os.path.join(current_dir, "../.."))
+
+    # Add project root to PYTHONPATH to ensure local iodata code is used
+    env = os.environ.copy()
+    current_pythonpath = env.get("PYTHONPATH", "")
+    env["PYTHONPATH"] = f"{project_root}:{current_pythonpath}"
+
+    subprocess.check_call([sys.executable, str(script_file)], cwd=tmp_path, env=env)
diff --git a/pyproject.toml b/pyproject.toml
index 2ad76494c..d524459be 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -53,6 +53,7 @@ dev = [
     "sphinx_autodoc_typehints",
     "sphinx-copybutton",
     "sympy",
+
 ]
 
 [project.urls]