diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index e9bb15ada..4a1ab3bc6 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -8,6 +8,12 @@ Removals:
 
 * Drop support for EOL Python 3.9. (:pull:`1263`)
 
+Fixes for requirements and markers:
+
+* Reject embedded whitespace in the URL of a direct-reference requirement, so a
+  newline can no longer split one requirement into two when the parsed result is
+  serialized. (:pull:`1379`)
+
 26.3 - 2026-08-03
 ~~~~~~~~~~~~~~~~~
 
diff --git a/src/packaging/_tokenizer.py b/src/packaging/_tokenizer.py
index 715842114..9249906ec 100644
--- a/src/packaging/_tokenizer.py
+++ b/src/packaging/_tokenizer.py
@@ -82,7 +82,7 @@ def __str__(self) -> str:
         re.VERBOSE | re.IGNORECASE,
     ),
     "AT": re.compile(r"\@"),
-    "URL": re.compile(r"[^ \t]+"),
+    "URL": re.compile(r"\S+"),
     "IDENTIFIER": re.compile(r"\b[a-zA-Z0-9][a-zA-Z0-9._-]*\b"),
     "VERSION_PREFIX_TRAIL": re.compile(r"\.\*"),
     "VERSION_LOCAL_LABEL_TRAIL": re.compile(r"\+[a-z0-9]+(?:[-_\.][a-z0-9]+)*"),
diff --git a/tests/test_requirements.py b/tests/test_requirements.py
index 8c0fd8ede..7e2e3643f 100644
--- a/tests/test_requirements.py
+++ b/tests/test_requirements.py
@@ -235,6 +235,7 @@ def test_file_url(self, url: str) -> None:
         [
             "name>=1",
             'name; python_version >= "3"',
+            "name @ https://example.com/name.whl",
         ],
     )
     def test_error_when_suffixed_with_line_break(
@@ -243,6 +244,14 @@ def test_error_when_suffixed_with_line_break(
         with pytest.raises(InvalidRequirement):
             Requirement(requirement + line_break)
 
+    @pytest.mark.parametrize("line_break", ["\n", "\r", "\r\n"])
+    def test_error_when_url_embeds_line_break(self, line_break: str) -> None:
+        # A line break inside the URL must not let the remainder be
+        # absorbed into the URL and later serialized as a second
+        # requirement line.
+        with pytest.raises(InvalidRequirement):
+            Requirement(f"name @ https://example.com/name.whl{line_break}evil==1")
+
     @pytest.mark.parametrize("whitespace", [" ", "\t", " \t"])
     def test_trailing_horizontal_whitespace(self, whitespace: str) -> None:
         assert Requirement("name>=1" + whitespace) == Requirement("name>=1")