diff --git a/.github/workflows/test_package.yml b/.github/workflows/test_package.yml
index 9fbbfa5..4518be0 100644
--- a/.github/workflows/test_package.yml
+++ b/.github/workflows/test_package.yml
@@ -69,6 +69,7 @@ jobs:
       - name: Test download h5 scripts exist
         run: |
           download_endf --help
+          download_jeff --help
           download_tendl --help
 
       - name: Test download chain files scripts exist
diff --git a/README.md b/README.md
index 609d98d..ddcfc61 100644
--- a/README.md
+++ b/README.md
@@ -106,10 +106,11 @@ A few categories of scripts are available:
 
 ### Download cross sections
 
-| Script name | Library | Release | Processed by |
-|-|-|-|-|
-| download_endf | ENDF/B | VII.1<br>VIII.0<br>VIII.1  | NNDC |
-| download_tendl | TENDL | 2019<br>2021  |  |
+| Script name | Library | Release | Processed by | Neutron temperatures (K) |
+|-|-|-|-|-|
+| download_endf | ENDF/B | VII.1<br>VIII.0<br>VIII.1  | NNDC | 250, 293.6, 600, 900, 1200, 2500 |
+| download_jeff | JEFF | 3.3<br>4.0 | OpenMC development team | 250, 293.6, 600, 900, 1200, 2500 |
+| download_tendl | TENDL | 2019<br>2021  |  | 293.6 |
 
 <!-- | Script name | Library | Release | Processed by | Download available | Downloads ACE files and convert to HDF5 | Downloads ENDF files and convert to HDF5 | Convert local ACE files |
 |-|-|-|-|-|-|-|-|
diff --git a/pyproject.toml b/pyproject.toml
index 942fab3..bb6bae7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -94,5 +94,6 @@ sample_sandy = "openmc_data.other.sample_sandy:main"
 reduce_chain = "openmc_data.depletion.reduce_chain:main"
 
 download_endf = "openmc_data.download.download_endf:main"
+download_jeff = "openmc_data.download.download_jeff:main"
 download_tendl = "openmc_data.download.download_tendl:main"
 download_chain = "openmc_data.download.download_chain:main"
diff --git a/src/openmc_data/download/download_jeff.py b/src/openmc_data/download/download_jeff.py
new file mode 100644
index 0000000..f66604b
--- /dev/null
+++ b/src/openmc_data/download/download_jeff.py
@@ -0,0 +1,54 @@
+#!/usr/bin/env python3
+
+"""
+Downloads official JEFF HDF5 libraries and cross_sections.xml from openmc.org
+for use with OpenMC. Includes JEFF neutron and thermal scattering data, plus
+ENDF/B photoatomic and atomic relaxation data (VIII.0 for JEFF 3.3, VIII.1 for 4.0).
+"""
+
+import argparse
+from pathlib import Path
+from urllib.parse import urljoin
+
+from openmc_data.urls_h5 import all_h5_release_details
+from openmc_data.utils import download, extract
+
+
+def main(argv=None):
+    """Download and optionally extract an official JEFF HDF5 library."""
+    parser = argparse.ArgumentParser(description=__doc__,
+                                     formatter_class=argparse.ArgumentDefaultsHelpFormatter)
+    parser.add_argument(
+        "-d", "--destination", type=Path, default=Path(),
+        help="Parent directory for jeff-<release>-download and jeff-<release>-hdf5",
+    )
+    parser.add_argument("-r", "--release", choices=["3.3", "4.0"], default="4.0",
+                        help="JEFF release")
+    for flag, default, help_text in (
+        ("download", True, "Download files listed on openmc.org"),
+        ("extract", True, "Extract archive files"),
+        ("cleanup", False, "Remove archives after successful extraction"),
+    ):
+        parser.add_argument("--" + flag, action="store_true", default=default, help=help_text)
+        parser.add_argument("--no-" + flag, dest=flag, action="store_false")
+    args = parser.parse_args(argv)
+
+    details = all_h5_release_details["jeff"][args.release]["neutron-photon"]
+    download_path = args.destination / f"jeff-{args.release}-download"
+    destination = args.destination / f"jeff-{args.release}-hdf5"
+
+    if args.download:
+        # Archive and extracted sizes are not recorded, so do not estimate them.
+        for filename in details["compressed_files"]:
+            download(urljoin(details["base_url"], filename), output_path=download_path)
+
+    if args.extract:
+        extract(
+            compressed_files=[download_path / f for f in details["compressed_files"]],
+            extraction_dir=destination,
+            del_compressed_file=args.cleanup,
+        )
+
+
+if __name__ == "__main__":
+    main()
\ No newline at end of file
diff --git a/src/openmc_data/urls_h5.py b/src/openmc_data/urls_h5.py
index ee52649..11b6676 100644
--- a/src/openmc_data/urls_h5.py
+++ b/src/openmc_data/urls_h5.py
@@ -25,6 +25,23 @@
             }
         },
     },
+    # Official OpenMC libraries: https://openmc.org/data/#official-data-libraries
+    # These include JEFF neutron/thermal data and ENDF/B photon supplements,
+    # not the older NEA-converted JEFF 3.3 library. Sizes are not recorded.
+    "jeff": {
+        "3.3": {
+            "neutron-photon": {
+                "base_url": "https://anl.box.com/shared/static/",
+                "compressed_files": ["4jwkvrr9pxlruuihcrgti75zde6g7bum.xz"],
+            }
+        },
+        "4.0": {
+            "neutron-photon": {
+                "base_url": "https://anl.box.com/shared/static/",
+                "compressed_files": ["fmcr8zoox1t6um0rwb2iw1nz72xue8ol.xz"],
+            }
+        },
+    },
     "tendl": {
         "2019": {
             "neutron": {
diff --git a/tests/test_download_jeff.py b/tests/test_download_jeff.py
new file mode 100644
index 0000000..27c03c1
--- /dev/null
+++ b/tests/test_download_jeff.py
@@ -0,0 +1,207 @@
+"""Offline tests: never download the multi-GB production libraries."""
+
+import argparse
+import importlib.util
+import io
+from pathlib import Path
+import shutil
+import socket
+import sys
+import tarfile
+from unittest.mock import Mock, call
+
+import pytest
+
+from openmc_data import utils
+from openmc_data.download import download_jeff
+from openmc_data.urls_h5 import all_h5_release_details
+
+
+ARCHIVES = {
+    "3.3": "4jwkvrr9pxlruuihcrgti75zde6g7bum.xz",
+    "4.0": "fmcr8zoox1t6um0rwb2iw1nz72xue8ol.xz",
+}
+BASE_URL = "https://anl.box.com/shared/static/"
+
+
+@pytest.fixture(autouse=True)
+def offline(monkeypatch, tmp_path):
+    monkeypatch.chdir(tmp_path)
+
+    def forbidden(*args, **kwargs):
+        pytest.fail("Network access and unverified size estimates are forbidden")
+
+    monkeypatch.setattr(socket, "create_connection", forbidden)
+    monkeypatch.setattr(socket.socket, "connect", forbidden)
+    monkeypatch.setattr(socket.socket, "connect_ex", forbidden)
+    monkeypatch.setattr(utils, "urlopen", forbidden)
+    monkeypatch.setattr(utils, "state_download_size", forbidden)
+    monkeypatch.setattr(download_jeff, "state_download_size", forbidden, raising=False)
+
+
+@pytest.fixture
+def operations(monkeypatch):
+    download = Mock()
+    extract = Mock()
+    monkeypatch.setattr(download_jeff, "download", download)
+    monkeypatch.setattr(download_jeff, "extract", extract)
+    return download, extract
+
+
+@pytest.mark.parametrize("release", ARCHIVES)
+def test_official_archive_metadata(release):
+    details = all_h5_release_details["jeff"][release]["neutron-photon"]
+    assert details["base_url"] == BASE_URL
+    assert details["compressed_files"] == [ARCHIVES[release]]
+    assert "compressed_file_size" not in details
+    assert "uncompressed_file_size" not in details
+
+
+def test_import_is_safe(monkeypatch, tmp_path, capsys):
+    forbidden = Mock(side_effect=AssertionError("Import must not perform work"))
+    monkeypatch.setattr(sys, "argv", ["unrelated-program", "--unrelated-option"])
+    monkeypatch.setattr(argparse.ArgumentParser, "parse_args", forbidden)
+    monkeypatch.setattr(utils, "download", forbidden)
+    monkeypatch.setattr(utils, "extract", forbidden)
+    spec = importlib.util.spec_from_file_location("jeff_import_test", download_jeff.__file__)
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+
+    assert callable(module.main)
+    forbidden.assert_not_called()
+    assert list(tmp_path.iterdir()) == []
+    assert capsys.readouterr().out == ""
+
+
+@pytest.mark.parametrize("release,options", [
+    ("4.0", []), ("3.3", ["-r", "3.3"]), ("4.0", ["--release", "4.0"]),
+])
+@pytest.mark.parametrize("destination", [None, "relative", "absolute"])
+@pytest.mark.parametrize("flags, download, extract, cleanup", [
+    ([], True, True, False),
+    (["--download", "--extract", "--cleanup"], True, True, True),
+    (["--no-download"], False, True, False),
+    (["--no-extract"], True, False, False),
+    (["--no-download", "--no-extract", "--cleanup"], False, False, True),
+    (["--cleanup", "--no-cleanup"], True, True, False),
+    (["--no-download", "--download", "--no-extract", "--extract"], True, True, False),
+    (["--download", "--no-download", "--extract", "--no-extract"], False, False, False),
+])
+def test_cli(operations, tmp_path, monkeypatch, release, options, destination,
+             flags, download, extract, cleanup):
+    root = Path("nested data/libraries") if destination else Path()
+    if destination == "absolute":
+        root = tmp_path / root
+    argv = [*options, *flags]
+    if destination:
+        argv += ["-d" if destination == "relative" else "--destination", str(root)]
+    monkeypatch.setattr(sys, "argv", ["download_jeff", *argv])
+    download_jeff.main()  # Exercise sys.argv as well as explicit argv in other tests.
+    archive = root / f"jeff-{release}-download" / ARCHIVES[release]
+    assert operations[0].call_args_list == ([call(BASE_URL + archive.name, output_path=archive.parent)] if download else [])
+    assert operations[1].call_args_list == ([call(
+        compressed_files=[archive], extraction_dir=root / f"jeff-{release}-hdf5",
+        del_compressed_file=cleanup)] if extract else [])
+    assert list(tmp_path.iterdir()) == []
+
+
+@pytest.mark.parametrize("argv,code", [(["--help"], 0)] + [
+    (["--release", value], 2) for value in ("3.2", "b8.1", "5.0")
+])
+def test_parser_exit(operations, tmp_path, capsys, argv, code):
+    with pytest.raises(SystemExit) as exc:
+        download_jeff.main(argv)
+    assert exc.value.code == code
+    output = capsys.readouterr()
+    expected = ("invalid choice", "3.3", "4.0") if code else (
+        "--release", "{3.3,4.0}", "--destination", "--download", "--no-download",
+        "--extract", "--no-extract", "--cleanup", "--no-cleanup", "default: 4.0")
+    assert all(text in (output.err if code else output.out) for text in expected)
+    assert not any(mock.called for mock in operations)
+    assert list(tmp_path.iterdir()) == []
+
+
+@pytest.fixture
+def tiny_archive(tmp_path):
+    """A real tar.xz archive with small, non-nuclear-data payloads."""
+    archive = tmp_path / "tiny.tar.xz"
+    payloads = {
+        "library/cross_sections.xml": b"<cross_sections />\n",
+        "library/neutron/payload.txt": b"test payload\n",
+    }
+    with tarfile.open(archive, "w:xz") as tar:
+        for name, contents in payloads.items():
+            member = tarfile.TarInfo(name)
+            member.size = len(contents)
+            tar.addfile(member, io.BytesIO(contents))
+    return archive, payloads
+
+
+@pytest.mark.parametrize("release", ARCHIVES)
+@pytest.mark.parametrize("download", [False, True])
+@pytest.mark.parametrize("cleanup", [False, True])
+def test_real_extraction(monkeypatch, tmp_path, tiny_archive, release, download, cleanup):
+    archive, payloads = tiny_archive
+    root = tmp_path / "custom" / "destination"
+    download_path = root / f"jeff-{release}-download"
+    compressed_file = download_path / ARCHIVES[release]
+
+    def copy_fixture(url, output_path):
+        assert url == BASE_URL + ARCHIVES[release]
+        assert output_path == download_path
+        output_path.mkdir(parents=True, exist_ok=True)
+        shutil.copyfile(archive, compressed_file)
+        return compressed_file
+
+    download_mock = Mock(side_effect=copy_fixture)
+    monkeypatch.setattr(download_jeff, "download", download_mock)
+    if not download:
+        copy_fixture(BASE_URL + ARCHIVES[release], download_path)
+    download_jeff.main([
+        "-r", release, "-d", str(root),
+        "--download" if download else "--no-download",
+        "--cleanup" if cleanup else "--no-cleanup",
+    ])
+
+    assert download_mock.call_count == int(download)
+    for name, contents in payloads.items():
+        assert (root / f"jeff-{release}-hdf5" / name).read_bytes() == contents
+    assert compressed_file.exists() is not cleanup
+    assert archive.exists()
+
+
+@pytest.mark.parametrize("release", ARCHIVES)
+def test_cleanup_without_extraction_keeps_archive(tmp_path, tiny_archive, release):
+    archive, _ = tiny_archive
+    download_path = tmp_path / f"jeff-{release}-download"
+    download_path.mkdir()
+    compressed_file = download_path / ARCHIVES[release]
+    shutil.copyfile(archive, compressed_file)
+
+    download_jeff.main(["-r", release, "--no-download", "--no-extract", "--cleanup"])
+
+    assert compressed_file.read_bytes() == archive.read_bytes()
+    assert not (tmp_path / f"jeff-{release}-hdf5").exists()
+
+
+@pytest.mark.parametrize("release", ARCHIVES)
+def test_failed_extraction_keeps_archive(tmp_path, release):
+    download_path = tmp_path / f"jeff-{release}-download"
+    download_path.mkdir()
+    compressed_file = download_path / ARCHIVES[release]
+    compressed_file.write_bytes(b"not an archive")
+
+    with pytest.raises(tarfile.ReadError):
+        download_jeff.main(["-r", release, "--no-download", "--cleanup"])
+
+    assert compressed_file.read_bytes() == b"not an archive"
+
+
+def test_failed_download_does_not_extract(operations):
+    download, extract = operations
+    download.side_effect = OSError("download failed")
+
+    with pytest.raises(OSError, match="download failed"):
+        download_jeff.main(["--cleanup"])
+
+    extract.assert_not_called()
\ No newline at end of file