diff --git a/rsconnect/environment.py b/rsconnect/environment.py
index 85bd1037..77ce406d 100644
--- a/rsconnect/environment.py
+++ b/rsconnect/environment.py
@@ -143,7 +143,7 @@ def create_python_environment(
         python_version_requirement = pyproject.detect_python_version_requirement(directory)
         _warn_on_missing_python_version(python_version_requirement)
 
-        _check_requirements_file(directory, requirements_file)
+        requirements_file = _check_requirements_file(directory, requirements_file)
 
         if python is not None:
             # TODO: Remove the option in a future release
@@ -314,7 +314,7 @@ def _warn_on_ignored_manifest(directory: str) -> None:
         warn_user("    Warning: the existing manifest.json file will not be used or considered.")
 
 
-def _check_requirements_file(directory: str, requirements_file: typing.Optional[str]) -> None:
+def _check_requirements_file(directory: str, requirements_file: typing.Optional[str]) -> typing.Optional[str]:
     """
     Verify that a requirements file exists inside the deployment directory.
 
@@ -324,13 +324,14 @@ def _check_requirements_file(directory: str, requirements_file: typing.Optional[
     if requirements_file is None:
         return
 
-    directory_path = pathlib.Path(directory)
-    requirements_file_path = directory_path / pathlib.Path(requirements_file)
-    if directory_path not in requirements_file_path.parents:
-        warn_user(
-            "    Warning: The requirements file '%s' is outside of the deployment directory.\n" % requirements_file,
-            fg="red",
-        )
+    directory_path = pathlib.Path(directory).resolve()
+    requirements_file_path = (directory_path / requirements_file).resolve()
+    try:
+        relative_path = requirements_file_path.relative_to(directory_path)
+    except ValueError:
+        raise RSConnectException(
+            "The requirements file '%s' is outside of the deployment directory '%s'." % (requirements_file, directory)
+        ) from None
 
     if not requirements_file_path.exists():
         raise RSConnectException(
@@ -339,6 +340,7 @@ def _check_requirements_file(directory: str, requirements_file: typing.Optional[
             "To have the requirements file generated using pip freeze, pass --force-generate."
             % (requirements_file, directory)
         )
+    return str(relative_path)
 
 
 def _warn_if_environment_directory(directory: typing.Union[str, pathlib.Path]) -> None:
diff --git a/tests/test_main.py b/tests/test_main.py
index 94a59c2f..f55589fb 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -2169,18 +2169,50 @@ def test_force_generate_requirements(self, tmp_path, existing_requirements):
         assert requirements.read_text().startswith("# requirements.txt generated by rsconnect-python")
         assert manifest["files"]["requirements.txt"]["checksum"] == file_checksum(requirements)
 
-    def test_explicit_requirements_file(self, tmp_path):
+    @pytest.mark.parametrize("path_kind", ["relative", "absolute", "normalized"])
+    def test_explicit_requirements_file(self, tmp_path, path_kind):
         (tmp_path / "app.py").write_text("app = object()\n")
-        requirements = tmp_path / "custom-requirements.txt"
+        (tmp_path / "dependencies").mkdir()
+        requirements = tmp_path / "dependencies" / "custom-requirements.txt"
         requirements.write_text("example-package==1.0\n")
+        relative_path = os.path.join("dependencies", requirements.name)
+        requirement_paths = {
+            "relative": relative_path,
+            "absolute": str(requirements),
+            "normalized": os.path.join("dependencies", "..", relative_path),
+        }
 
         runner = CliRunner()
-        result = runner.invoke(cli, ["write-manifest", "api", "-r", requirements.name, str(tmp_path)])
+        result = runner.invoke(cli, ["write-manifest", "api", "-r", requirement_paths[path_kind], str(tmp_path)])
         assert result.exit_code == 0, result.output
 
         manifest = json.loads((tmp_path / "manifest.json").read_text())
         assert requirements.read_text() == "example-package==1.0\n"
-        assert manifest["files"][requirements.name]["checksum"] == file_checksum(requirements)
+        assert manifest["python"]["package_manager"]["package_file"] == relative_path
+        assert manifest["files"][relative_path]["checksum"] == file_checksum(requirements)
+        assert set(manifest["files"]) == {"app.py", relative_path}
+        assert f"{relative_path} already exists and will not be overwritten" in result.output
+
+    @pytest.mark.parametrize("path_kind", ["absolute", "relative", "symlink"])
+    def test_requirements_file_outside_project_is_rejected(self, tmp_path, path_kind):
+        project = tmp_path / "project"
+        project.mkdir()
+        (project / "app.py").write_text("app = object()\n")
+        outside = tmp_path / "requirements.txt"
+        outside.write_text("example-package==1.0\n")
+        if path_kind == "symlink":
+            (project / "linked.txt").symlink_to(outside)
+        requirement_paths = {
+            "absolute": str(outside),
+            "relative": os.path.join("..", outside.name),
+            "symlink": "linked.txt",
+        }
+
+        result = CliRunner().invoke(cli, ["write-manifest", "api", "-r", requirement_paths[path_kind], str(project)])
+
+        assert result.exit_code != 0
+        assert "outside of the deployment directory" in result.output
+        assert not (project / "manifest.json").exists()
 
 
 class TestWriteManifestExcludeRenv: