diff --git a/crates/ruff_linter/resources/test/fixtures/pyupgrade/UP045.py b/crates/ruff_linter/resources/test/fixtures/pyupgrade/UP045.py
index 904c1510ea..13be285aa0 100644
--- a/crates/ruff_linter/resources/test/fixtures/pyupgrade/UP045.py
+++ b/crates/ruff_linter/resources/test/fixtures/pyupgrade/UP045.py
@@ -42,3 +42,8 @@ class ServiceRefOrValue:
 # Regression test for: https://github.com/astral-sh/ruff/issues/7201
 class ServiceRefOrValue:
     service_specification: Optional[str]is not True = None
+
+
+# Test for: https://github.com/astral-sh/ruff/issues/18508
+# Optional[None] should not be offered a fix
+foo: Optional[None] = None
diff --git a/crates/ruff_linter/src/rules/pyupgrade/rules/use_pep604_annotation.rs b/crates/ruff_linter/src/rules/pyupgrade/rules/use_pep604_annotation.rs
index b1b888cf4c..bc49d8df08 100644
--- a/crates/ruff_linter/src/rules/pyupgrade/rules/use_pep604_annotation.rs
+++ b/crates/ruff_linter/src/rules/pyupgrade/rules/use_pep604_annotation.rs
@@ -100,8 +100,8 @@ impl Violation for NonPEP604AnnotationUnion {
 /// ```
 ///
 /// ## Fix safety
-/// This rule's fix is marked as unsafe, as it may lead to runtime errors when
-/// alongside libraries that rely on runtime type annotations, like Pydantic,
+/// This rule's fix is marked as unsafe, as it may lead to runtime errors
+/// using libraries that rely on runtime type annotations, like Pydantic,
 /// on Python versions prior to Python 3.10. It may also lead to runtime errors
 /// in unusual and likely incorrect type annotations where the type does not
 /// support the `|` operator.
@@ -138,7 +138,8 @@ pub(crate) fn non_pep604_annotation(
     // lead to invalid syntax.
     let fixable = checker.semantic().in_type_definition()
         && !checker.semantic().in_complex_string_type_definition()
-        && is_allowed_value(slice);
+        && is_allowed_value(slice)
+        && !is_optional_none(operator, slice);
 
     let applicability = if checker.target_version() >= PythonVersion::PY310 {
         Applicability::Safe
@@ -276,3 +277,8 @@ fn is_allowed_value(expr: &Expr) -> bool {
         | Expr::IpyEscapeCommand(_) => false,
     }
 }
+
+/// Return `true` if this is an `Optional[None]` annotation.
+fn is_optional_none(operator: Pep604Operator, slice: &Expr) -> bool {
+    matches!(operator, Pep604Operator::Optional) && matches!(slice, Expr::NoneLiteral(_))
+}
diff --git a/crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP045.py.snap b/crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP045.py.snap
index 3579871395..9e78420b08 100644
--- a/crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP045.py.snap
+++ b/crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP045.py.snap
@@ -1,6 +1,5 @@
 ---
 source: crates/ruff_linter/src/rules/pyupgrade/mod.rs
-snapshot_kind: text
 ---
 UP045.py:5:10: UP007 [*] Use `X | Y` for type annotations
   |
@@ -149,3 +148,15 @@ UP045.py:44:28: UP007 [*] Use `X | Y` for type annotations
 43 43 | class ServiceRefOrValue:
 44    |-    service_specification: Optional[str]is not True = None
    44 |+    service_specification: str | None is not True = None
+45 45 | 
+46 46 | 
+47 47 | # Test for: https://github.com/astral-sh/ruff/issues/18508
+
+UP045.py:49:6: UP007 Use `X | Y` for type annotations
+   |
+47 | # Test for: https://github.com/astral-sh/ruff/issues/18508
+48 | # Optional[None] should not be offered a fix
+49 | foo: Optional[None] = None
+   |      ^^^^^^^^^^^^^^ UP007
+   |
+   = help: Convert to `X | Y`