diff --git a/CHANGES/13168.bugfix.rst b/CHANGES/13168.bugfix.rst
new file mode 100644
index 00000000000..90051787e6d
--- /dev/null
+++ b/CHANGES/13168.bugfix.rst
@@ -0,0 +1,4 @@
+Enforced the ``__Secure-`` and ``__Host-`` cookie name prefixes
+(RFC 6265bis, section 4.1.3) in :class:`~aiohttp.CookieJar`, so a
+prefixed cookie that is not host-locked and secure is no longer stored
+-- by :user:`arshsmith1`.
diff --git a/aiohttp/cookiejar.py b/aiohttp/cookiejar.py
index 8e945f3470d..82262fbb2d6 100644
--- a/aiohttp/cookiejar.py
+++ b/aiohttp/cookiejar.py
@@ -337,6 +337,21 @@ def update_cookies(self, cookies: LooseCookies, response_url: URL = URL()) -> No
                 tmp[name] = cookie  # type: ignore[assignment]
                 cookie = tmp[name]
 
+            # RFC 6265bis 4.1.3: enforce the __Secure- and __Host- name
+            # prefixes so a related-origin (sibling subdomain) or on-path party
+            # cannot forge a cookie the target server trusts as host-locked and
+            # secure. Only applied to cookies from an actual response; a jar
+            # seeded programmatically has no response host to check against.
+            if hostname is not None:
+                lower_name = name.lower()
+                if lower_name.startswith(("__secure-", "__host-")):
+                    if not cookie["secure"] or not self._is_secure_origin(response_url):
+                        continue
+                    if lower_name.startswith("__host-") and (
+                        cookie["domain"] or cookie["path"] != "/"
+                    ):
+                        continue
+
             domain = cookie["domain"]
 
             # ignore domains with trailing dots
@@ -495,6 +510,15 @@ def _build_morsel(self, cookie: Morsel[str]) -> Morsel[str]:
         morsel.__setstate__({"key": cookie.key, "value": value, "coded_value": coded_value})  # type: ignore[attr-defined]
         return morsel
 
+    def _is_secure_origin(self, url: URL) -> bool:
+        """Whether cookies from url should be treated as a secure origin."""
+        if url.scheme in ("https", "wss"):
+            return True
+        if self._treat_as_secure_origin:
+            with contextlib.suppress(ValueError):
+                return url.origin() in self._treat_as_secure_origin
+        return False
+
     @staticmethod
     def _is_domain_match(domain: str, hostname: str) -> bool:
         """Implements domain matching adhering to RFC 6265."""
diff --git a/tests/test_cookiejar.py b/tests/test_cookiejar.py
index 0c24df71c0b..7872c4f3f60 100644
--- a/tests/test_cookiejar.py
+++ b/tests/test_cookiejar.py
@@ -421,6 +421,65 @@ async def test_ignore_domain_ending_with_dot() -> None:
     assert cookies_sent.output(header="Cookie:") == ""
 
 
+@pytest.mark.parametrize(
+    ("set_cookie", "url"),
+    [
+        # __Host- with a Domain attribute is not host-only.
+        (
+            "__Host-sid=x; Domain=example.com; Path=/; Secure",
+            "https://sub.example.com/",
+        ),
+        # __Host- without Path=/.
+        ("__Host-sid=x; Path=/admin; Secure", "https://example.com/admin"),
+        # __Host- without Secure.
+        ("__Host-sid=x; Path=/", "https://example.com/"),
+        # Prefixes must not be accepted over an insecure origin.
+        ("__Host-sid=x; Path=/; Secure", "http://example.com/"),
+        ("__Secure-sid=x; Secure", "http://example.com/"),
+        # __Secure- without the Secure attribute.
+        ("__Secure-sid=x", "https://example.com/"),
+        # Prefix match is case-insensitive, so casing cannot bypass it.
+        (
+            "__host-sid=x; Domain=example.com; Path=/; Secure",
+            "https://sub.example.com/",
+        ),
+    ],
+)
+async def test_cookie_prefix_rejected(set_cookie: str, url: str) -> None:
+    jar = CookieJar()
+    jar.update_cookies(SimpleCookie(set_cookie), URL(url))
+    assert len(jar) == 0
+
+
+@pytest.mark.parametrize(
+    ("set_cookie", "url"),
+    [
+        ("__Host-sid=x; Path=/; Secure", "https://example.com/"),
+        ("__Secure-sid=x; Secure", "https://example.com/"),
+        ("__Secure-sid=x; Domain=example.com; Secure", "https://www.example.com/"),
+    ],
+)
+async def test_cookie_prefix_accepted(set_cookie: str, url: str) -> None:
+    jar = CookieJar()
+    jar.update_cookies(SimpleCookie(set_cookie), URL(url))
+    assert len(jar) == 1
+
+
+async def test_cookie_prefix_programmatic_seed_unaffected() -> None:
+    # Seeding the jar without a response URL is a trusted path and keeps working.
+    jar = CookieJar()
+    jar.update_cookies(SimpleCookie("__Host-seed=x; Path=/; Secure"))
+    assert "__Host-seed" in jar.filter_cookies(URL("https://any.example/"))
+
+
+async def test_cookie_prefix_treat_as_secure_origin() -> None:
+    jar = CookieJar(treat_as_secure_origin=URL("http://example.com"))
+    jar.update_cookies(
+        SimpleCookie("__Host-sid=x; Path=/; Secure"), URL("http://example.com/")
+    )
+    assert "__Host-sid" in jar.filter_cookies(URL("http://example.com/"))
+
+
 class TestCookieJarSafe:
     @pytest.fixture(autouse=True)
     def setup_cookies(