diff --git a/docs/changelog/2373.bugfix.rst b/docs/changelog/2373.bugfix.rst
new file mode 100644
index 00000000..1fb3436f
--- /dev/null
+++ b/docs/changelog/2373.bugfix.rst
@@ -0,0 +1,1 @@
+Allow ``--hash`` to be specified in requirements.txt files. - by :user:`masenf`.
diff --git a/src/tox/tox_env/python/pip/req/args.py b/src/tox/tox_env/python/pip/req/args.py
index 19bded34..23f98ec3 100644
--- a/src/tox/tox_env/python/pip/req/args.py
+++ b/src/tox/tox_env/python/pip/req/args.py
@@ -18,10 +18,10 @@ class _OurArgumentParser(ArgumentParser):
         raise ValueError(msg)
 
 
-def build_parser(cli_only: bool) -> ArgumentParser:
+def build_parser() -> ArgumentParser:
     parser = _OurArgumentParser(add_help=False, prog="", allow_abbrev=False)
     _global_options(parser)
-    _req_options(parser, cli_only)
+    _req_options(parser)
     return parser
 
 
@@ -47,11 +47,10 @@ def _global_options(parser: ArgumentParser) -> None:
     )
 
 
-def _req_options(parser: ArgumentParser, cli_only: bool) -> None:
+def _req_options(parser: ArgumentParser) -> None:
     parser.add_argument("--install-option", action=AddSortedUniqueAction)
     parser.add_argument("--global-option", action=AddSortedUniqueAction)
-    if not cli_only:
-        parser.add_argument("--hash", action=AddSortedUniqueAction, type=_validate_hash)
+    parser.add_argument("--hash", action=AddSortedUniqueAction, type=_validate_hash)
 
 
 _HASH = re.compile(r"sha(256:[a-f0-9]{64}|384:[a-f0-9]{96}|512:[a-f0-9]{128})")
diff --git a/src/tox/tox_env/python/pip/req/file.py b/src/tox/tox_env/python/pip/req/file.py
index 4ad4ae06..df11ebe5 100644
--- a/src/tox/tox_env/python/pip/req/file.py
+++ b/src/tox/tox_env/python/pip/req/file.py
@@ -156,7 +156,7 @@ class RequirementsFile:
     @property
     def _parser(self) -> ArgumentParser:
         if self._parser_private is None:
-            self._parser_private = build_parser(False)
+            self._parser_private = build_parser()
         return self._parser_private
 
     def _ensure_requirements_parsed(self) -> None:
diff --git a/src/tox/tox_env/python/pip/req_file.py b/src/tox/tox_env/python/pip/req_file.py
index 1f8754a8..91202345 100644
--- a/src/tox/tox_env/python/pip/req_file.py
+++ b/src/tox/tox_env/python/pip/req_file.py
@@ -1,14 +1,17 @@
 from __future__ import annotations
 
 import re
-from argparse import ArgumentParser
+from argparse import Namespace
 from pathlib import Path
 
-from .req.args import build_parser
-from .req.file import ReqFileLines, RequirementsFile
+from .req.file import ParsedRequirement, ReqFileLines, RequirementsFile
 
 
 class PythonDeps(RequirementsFile):
+    # these options are valid in requirements.txt, but not via pip cli and
+    # thus cannot be used in the testenv `deps` list
+    _illegal_options = ["hash"]
+
     def __init__(self, raw: str, root: Path):
         super().__init__(root / "tox.ini", constraint=False)
         self._raw = self._normalize_raw(raw)
@@ -28,12 +31,6 @@ class PythonDeps(RequirementsFile):
                 line = f"{line[0:2]} {line[2:]}"
             yield at, line
 
-    @property
-    def _parser(self) -> ArgumentParser:
-        if self._parser_private is None:
-            self._parser_private = build_parser(cli_only=True)  # e.g. no --hash for cli only
-        return self._parser_private
-
     def lines(self) -> list[str]:
         return self._raw.splitlines()
 
@@ -68,6 +65,20 @@ class PythonDeps(RequirementsFile):
         raw = f"{adjusted}\n" if raw.endswith("\\\n") else adjusted  # preserve trailing newline if input has it
         return raw
 
+    def _parse_requirements(self, opt: Namespace, recurse: bool) -> list[ParsedRequirement]:
+        # check for any invalid options in the deps list
+        # (requirements recursively included from other files are not checked)
+        requirements = super()._parse_requirements(opt, recurse)
+        for r in requirements:
+            if r.from_file != str(self.path):
+                continue
+            for illegal_option in self._illegal_options:
+                if r.options.get(illegal_option):
+                    raise ValueError(
+                        f"Cannot use --{illegal_option} in deps list, it must be in requirements file. ({r})",
+                    )
+        return requirements
+
     def unroll(self) -> tuple[list[str], list[str]]:
         if self._unroll is None:
             opts_dict = vars(self.options)

diff --git a/tests/tox_env/python/pip/test_req_file.py b/tests/tox_env/python/pip/test_req_file.py
index 41908f96..66a0db9a 100644
--- a/tests/tox_env/python/pip/test_req_file.py
+++ b/tests/tox_env/python/pip/test_req_file.py
@@ -14,3 +14,31 @@ def test_legacy_requirement_file(tmp_path: Path, legacy_flag: str) -> None:
     assert python_deps.as_root_args == [legacy_flag, "a.txt"]
     assert vars(python_deps.options) == {}
     assert [str(i) for i in python_deps.requirements] == ["b" if legacy_flag == "-r" else "-c b"]
+
+
+def test_deps_with_hash(tmp_path: Path) -> None:
+    """deps with --hash should raise an exception."""
+    python_deps = PythonDeps(
+        raw="foo==1 --hash sha256:97a702083b0d906517b79672d8501eee470d60ae55df0fa9d4cfba56c7f65a82",
+        root=tmp_path,
+    )
+    with pytest.raises(ValueError, match="Cannot use --hash in deps list"):
+        _ = python_deps.requirements
+
+
+def test_deps_with_requirements_with_hash(tmp_path: Path) -> None:
+    """deps can point to a requirements file that has --hash."""
+    exp_hash = "sha256:97a702083b0d906517b79672d8501eee470d60ae55df0fa9d4cfba56c7f65a82"
+    requirements = tmp_path / "requirements.txt"
+    requirements.write_text(
+        f"foo==1 --hash {exp_hash}",
+    )
+    python_deps = PythonDeps(
+        raw="-r requirements.txt",
+        root=tmp_path,
+    )
+    assert len(python_deps.requirements) == 1
+    parsed_req = python_deps.requirements[0]
+    assert str(parsed_req.requirement) == "foo==1"
+    assert parsed_req.options == {"hash": [exp_hash]}
+    assert parsed_req.from_file == str(requirements)

tox4: fails to process requirement files with --hash
There is a regression on tox4 where it fails to parse requirement files that contain hashes. While these are not very popular they are still the recommended for security reasons as they protect against potential hacks on pypi registry.

Example of file that causes tox4 to fail, while it works fine with tox3: https://github.com/ansible/ansible-language-server/blob/v0.5.0/docs/requirements.txt
 
```
During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/ssbarnea/.pyenv/versions/3.10.2/lib/python3.10/site-packages/tox/session/cmd/run/single.py", line 45, in _evaluate
    tox_env.setup()
  File "/Users/ssbarnea/.pyenv/versions/3.10.2/lib/python3.10/site-packages/tox/tox_env/api.py", line 226, in setup
    self._setup_env()
  File "/Users/ssbarnea/.pyenv/versions/3.10.2/lib/python3.10/site-packages/tox/tox_env/python/runner.py", line 91, in _setup_env
    self._install_deps()
  File "/Users/ssbarnea/.pyenv/versions/3.10.2/lib/python3.10/site-packages/tox/tox_env/python/runner.py", line 95, in _install_deps
    self.installer.install(requirements_file, PythonRun.__name__, "deps")
  File "/Users/ssbarnea/.pyenv/versions/3.10.2/lib/python3.10/site-packages/tox/tox_env/python/pip/pip_install.py", line 84, in install
    self._install_requirement_file(arguments, section, of_type)
  File "/Users/ssbarnea/.pyenv/versions/3.10.2/lib/python3.10/site-packages/tox/tox_env/python/pip/pip_install.py", line 95, in _install_requirement_file
    raise HandledError(f"{exception} for tox env py within deps")
tox.report.HandledError: unrecognized arguments: --hash=sha256:446438bdcca0e05bd45ea2de1668c1d9b032e1a9154c2c259092d77031ddd359 --hash=sha256:a661d72d58e6ea8a57f7a86e37d86716863ee5e92788398526d58b26a4e4dc02 for tox env py within deps
```

It should be remarked that these files are produced by pip-compile (pip-tools).


Note: I temporary removed the hashes from the lock file but we cannot really ignore this issue.
tests/tox_env/python/pip/test_req_file.py::test_deps_with_hash tests/tox_env/python/pip/test_req_file.py::test_deps_with_requirements_with_hash
tests/tox_env/python/pip/test_req_file.py::test_legacy_requirement_file[-r] tests/tox_env/python/pip/test_req_file.py::test_legacy_requirement_file[-c]