diff --git a/docs/source/release-notes/unreleased.rst b/docs/source/release-notes/unreleased.rst
index 09b03b4..362003a 100644
--- a/docs/source/release-notes/unreleased.rst
+++ b/docs/source/release-notes/unreleased.rst
@@ -1,6 +1,7 @@
 2.x.y - 202z-aa-bb
 ------------------
 
-- *Add Items here*
+- Reject colons in the first path segment when validating a relative
+  reference without a scheme.
 
 .. links below here
diff --git a/src/rfc3986/_mixin.py b/src/rfc3986/_mixin.py
index b03b772..9a93de8 100644
--- a/src/rfc3986/_mixin.py
+++ b/src/rfc3986/_mixin.py
@@ -221,7 +221,9 @@ def path_is_valid(self, require: bool = False) -> bool:
             "This method will be eventually removed.",
             DeprecationWarning,
         )
-        return validators.path_is_valid(self.path, require)
+        return validators._path_is_valid_for_scheme(
+            self.path, self.scheme, require
+        )
 
     def query_is_valid(self, require: bool = False) -> bool:
         """Determine if the query component is valid.
diff --git a/src/rfc3986/validators.py b/src/rfc3986/validators.py
index d68f16b..554c21c 100644
--- a/src/rfc3986/validators.py
+++ b/src/rfc3986/validators.py
@@ -371,6 +371,17 @@ def path_is_valid(path: t.Optional[str], require: bool = False) -> bool:
     return is_valid(path, misc.PATH_MATCHER, require)
 
 
+def _path_is_valid_for_scheme(
+    path: t.Optional[str],
+    scheme: t.Optional[str],
+    require: bool = False,
+) -> bool:
+    """Apply the path-noscheme restriction to relative references."""
+    if not scheme and path and ":" in path.split("/", 1)[0]:
+        return False
+    return path_is_valid(path, require)
+
+
 def query_is_valid(query: t.Optional[str], require: bool = False) -> bool:
     """Determine if the query component is valid.
 
@@ -469,8 +480,12 @@ def ensure_components_are_valid(
             # https://bitbucket.org/ned/coveragepy/issues/198/continue-marked-as-not-covered
             continue  # nocov: Python 2.7, 3.3, 3.4
 
-        validator = _COMPONENT_VALIDATORS[component]
-        if not validator(getattr(uri, component)):
+        if component == "path":
+            valid = _path_is_valid_for_scheme(uri.path, uri.scheme)
+        else:
+            validator = _COMPONENT_VALIDATORS[component]
+            valid = validator(getattr(uri, component))
+        if not valid:
             invalid_components.add(component)
 
     if invalid_components:
diff --git a/tests/test_relative_path_validation.py b/tests/test_relative_path_validation.py
new file mode 100644
index 0000000..661c699
--- /dev/null
+++ b/tests/test_relative_path_validation.py
@@ -0,0 +1,38 @@
+import pytest
+
+from rfc3986 import exceptions
+from rfc3986 import uri_reference
+from rfc3986 import validators
+
+
+@pytest.mark.parametrize("text", [".://", "1:a", ":a", "a_b:c", "a%20b:c"])
+def test_relative_first_segment_cannot_contain_colon(text):
+    reference = uri_reference(text)
+    assert reference.scheme is None
+    with pytest.raises(exceptions.InvalidComponentsError):
+        validators.Validator().check_validity_of("path").validate(reference)
+    assert not reference.path_is_valid()
+    assert not reference.is_valid()
+
+
+@pytest.mark.parametrize(
+    "text",
+    ["./a:b", "/a:b", "a/b:c", "a%3Ab", "a:b", "urn:a:b", "", "?q=a:b"],
+)
+def test_colons_remain_valid_outside_relative_first_segment(text):
+    reference = uri_reference(text)
+    validators.Validator().check_validity_of("path").validate(reference)
+    assert reference.path_is_valid()
+    assert reference.is_valid()
+
+
+def test_component_validation_does_not_infer_uri_context():
+    assert validators.path_is_valid("a:b")
+
+
+def test_path_validation_remains_opt_in():
+    validators.Validator().validate(uri_reference(".://"))
+
+
+def test_required_path_still_rejects_missing_path():
+    assert not uri_reference("https://example.com").path_is_valid(require=True)