diff --git a/src/poetry/console/commands/group_command.py b/src/poetry/console/commands/group_command.py
index d876884fe55..86c031fd1ed 100644
--- a/src/poetry/console/commands/group_command.py
+++ b/src/poetry/console/commands/group_command.py
@@ -103,6 +103,18 @@ def activated_groups(self) -> set[NormalizedName]:
             key: {canonicalize_name(group) for group in key_groups}
             for key, key_groups in groups.items()
         }
+        # Warn if any group name was normalized (deduplicate warnings)
+        warned: set[str] = set()
+        for key_groups in groups.values():
+            for group in key_groups:
+                normalized = canonicalize_name(group)
+                if normalized != group and group not in warned:
+                    warned.add(group)
+                    self.line_error(
+                        f"<warning>Group '{group}' was normalized to '{normalized}'."
+                        " Consider updating your command to use the normalized name.</warning>"
+                    )
+
         norm_default_groups = {canonicalize_name(name) for name in self.default_groups}
 
         return norm_groups["only"] or norm_default_groups.union(
@@ -121,7 +133,9 @@ def _validate_group_options(self, group_options: dict[str, set[str]]) -> None:
         invalid_options = defaultdict(set)
         for opt, groups in group_options.items():
             for group in groups:
-                if not self.poetry.package.has_dependency_group(group):
+                if not self.poetry.package.has_dependency_group(
+                    canonicalize_name(group)
+                ):
                     invalid_options[group].add(opt)
         if invalid_options:
             message_parts = []
diff --git a/tests/console/commands/test_install.py b/tests/console/commands/test_install.py
index d5590dd77e1..155cd5bdde0 100644
--- a/tests/console/commands/test_install.py
+++ b/tests/console/commands/test_install.py
@@ -396,6 +396,63 @@ def test_invalid_groups_with_without_only(
                 )
 
 
+@pytest.mark.parametrize(
+    ("option", "group_input", "normalized"),
+    [
+        ("--with", "Foo", "foo"),
+        ("--without", "Foo", "foo"),
+        ("--only", "Foo", "foo"),
+        ("--with", "FOO", "foo"),
+        ("--with", "bim", "bim"),
+    ],
+)
+def test_non_normalized_group_name_emits_warning(
+    tester: CommandTester,
+    mocker: MockerFixture,
+    option: str,
+    group_input: str,
+    normalized: str,
+) -> None:
+    """
+    A warning is emitted when a group name passed via --with/--without/--only
+    differs from its normalized form (e.g. 'Foo' instead of 'foo').
+    """
+    assert isinstance(tester.command, InstallerCommand)
+    mocker.patch.object(tester.command.installer, "run", return_value=0)
+
+    tester.execute(f"{option} {group_input}")
+    error_output = tester.io.fetch_error()
+
+    if group_input != normalized:
+        assert "was normalized to" in error_output
+        assert f"'{group_input}'" in error_output
+        assert f"'{normalized}'" in error_output
+    else:
+        assert "was normalized to" not in error_output
+
+
+def test_normalized_group_name_does_not_raise(
+    tester: CommandTester,
+    mocker: MockerFixture,
+) -> None:
+    """
+    Passing a non-normalized but valid group name (e.g. 'Foo' for group 'foo')
+    should not raise a GroupNotFoundError.
+    """
+    assert isinstance(tester.command, InstallerCommand)
+    mocker.patch.object(tester.command.installer, "run", return_value=0)
+
+    # Should not raise even though 'Foo' != 'foo', because they normalize to the same name
+    # Use --no-root to avoid editable build failure in the test project fixture
+    tester.execute("--with Foo --no-root")
+    error_output = tester.io.fetch_error()
+    assert tester.status_code == 0
+    assert "GroupNotFoundError" not in error_output
+    assert "not found" not in error_output
+    assert "was normalized to" in error_output
+    assert "'Foo'" in error_output
+
+
 def test_dry_run_populates_installer(
     tester: CommandTester, mocker: MockerFixture
 ) -> None: