diff --git a/crates/ruff_linter/resources/test/fixtures/pyupgrade/UP007.py b/crates/ruff_linter/resources/test/fixtures/pyupgrade/UP007.py
index 5f23a92af2e8b..af1c1ca35b3a4 100644
--- a/crates/ruff_linter/resources/test/fixtures/pyupgrade/UP007.py
+++ b/crates/ruff_linter/resources/test/fixtures/pyupgrade/UP007.py
@@ -114,3 +114,8 @@ class ServiceRefOrValue:
 class Collection(Protocol[*_B0]):
     def __iter__(self) -> Iterator[Union[*_B0]]:
         ...
+
+
+# Regression test for: https://github.com/astral-sh/ruff/issues/8609
+def f(x: Union[int, str, bytes]) -> 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 73f1075140402..1823e116d7fec 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
@@ -147,10 +147,10 @@ fn union(elts: &[Expr]) -> Expr {
         }),
         [Expr::Tuple(ast::ExprTuple { elts, .. })] => union(elts),
         [elt] => elt.clone(),
-        [elt, rest @ ..] => Expr::BinOp(ast::ExprBinOp {
-            left: Box::new(union(&[elt.clone()])),
+        [rest @ .., elt] => Expr::BinOp(ast::ExprBinOp {
+            left: Box::new(union(rest)),
             op: Operator::BitOr,
-            right: Box::new(union(rest)),
+            right: Box::new(union(&[elt.clone()])),
             range: TextRange::default(),
         }),
     }
diff --git a/crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP007.py.snap b/crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP007.py.snap
index 4c8f7cc2c9b58..77252939a755b 100644
--- a/crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP007.py.snap
+++ b/crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP007.py.snap
@@ -50,7 +50,7 @@ UP007.py:14:10: UP007 [*] Use `X | Y` for type annotations
 12 12 | 
 13 13 | 
 14    |-def f(x: Union[str, int, Union[float, bytes]]) -> None:
-   14 |+def f(x: str | (int | Union[float, bytes])) -> None:
+   14 |+def f(x: str | int | Union[float, bytes]) -> None:
 15 15 |     ...
 16 16 | 
 17 17 | 
@@ -176,7 +176,7 @@ UP007.py:38:11: UP007 [*] Use `X | Y` for type annotations
 36 36 | 
 37 37 | 
 38    |-def f(x: "Union[str, int, Union[float, bytes]]") -> None:
-   38 |+def f(x: "str | (int | Union[float, bytes])") -> None:
+   38 |+def f(x: "str | int | Union[float, bytes]") -> None:
 39 39 |     ...
 40 40 | 
 41 41 | 
@@ -414,4 +414,21 @@ UP007.py:110:28: UP007 [*] Use `X | Y` for type annotations
 112 112 | 
 113 113 | # Regression test for: https://github.com/astral-sh/ruff/issues/7452
 
+UP007.py:120:10: UP007 [*] Use `X | Y` for type annotations
+    |
+119 | # Regression test for: https://github.com/astral-sh/ruff/issues/8609
+120 | def f(x: Union[int, str, bytes]) -> None:
+    |          ^^^^^^^^^^^^^^^^^^^^^^ UP007
+121 |     ...
+    |
+    = help: Convert to `X | Y`
+
+ℹ Unsafe fix
+117 117 | 
+118 118 | 
+119 119 | # Regression test for: https://github.com/astral-sh/ruff/issues/8609
+120     |-def f(x: Union[int, str, bytes]) -> None:
+    120 |+def f(x: int | str | bytes) -> None:
+121 121 |     ...
+