diff --git a/crates/oxc_mangler/src/lib.rs b/crates/oxc_mangler/src/lib.rs
index b27b26c9840c0..81df3a8d7fd27 100644
--- a/crates/oxc_mangler/src/lib.rs
+++ b/crates/oxc_mangler/src/lib.rs
@@ -338,11 +338,41 @@ impl<'t> Mangler<'t> {
         //
         // TODO: This is conservative, ideally, we would reserve names per-slot.
         let mut eval_reserved_names: FxHashSet<&str> = FxHashSet::default();
+
+        // Annex B.3.2.1: In sloppy mode, function declarations inside block scopes create
+        // an implicit `var`-like assignment in the enclosing function scope at block exit.
+        // If such a function gets the same mangled name as an outer `var`, the Annex B
+        // assignment overwrites it. To prevent this, hoist these functions to the enclosing
+        // var scope for slot assignment — the same way `var` is hoisted by oxc_semantic.
+        let mut annex_b_hoisted: FxHashMap<usize, Vec<'_, SymbolId>> = FxHashMap::default();
+        let mut annex_b_skip = BitSet::new_in(scoping.symbols_len(), temp_allocator);
+        for (scope_id, bindings) in scoping.iter_bindings() {
+            let flags = scoping.scope_flags(scope_id);
+            if flags.is_var() || flags.is_strict_mode() {
+                continue;
+            }
+            for &symbol_id in bindings.values() {
+                if !scoping.symbol_flags(symbol_id).is_function() {
+                    continue;
+                }
+                for ancestor_id in scoping.scope_ancestors(scope_id).skip(1) {
+                    if scoping.scope_flags(ancestor_id).is_var() {
+                        annex_b_hoisted
+                            .entry(ancestor_id.index())
+                            .or_insert_with(|| Vec::new_in(temp_allocator))
+                            .push(symbol_id);
+                        annex_b_skip.set_bit(symbol_id.index());
+                        break;
+                    }
+                }
+            }
+        }
+
         // Walk down the scope tree and assign a slot number for each symbol.
         // It is possible to do this in a loop over the symbol list,
         // but walking down the scope tree seems to generate a better code.
         for (scope_id, bindings) in scoping.iter_bindings() {
-            if bindings.is_empty() {
+            if bindings.is_empty() && !annex_b_hoisted.contains_key(&scope_id.index()) {
                 continue;
             }
             // Scopes with direct eval: collect binding names as reserved (they can be
@@ -351,16 +381,34 @@ impl<'t> Mangler<'t> {
                 for (name, _) in bindings {
                     eval_reserved_names.insert(name.as_str());
                 }
+                if let Some(hoisted) = annex_b_hoisted.get(&scope_id.index()) {
+                    for &symbol_id in hoisted {
+                        eval_reserved_names.insert(scoping.symbol_name(symbol_id));
+                    }
+                }
                 continue;
             }
 
-            // Sort `bindings` in declaration order.
+            // Collect bindings in declaration order.
+            // Annex B functions are skipped here — they're processed with their var scope.
             tmp_bindings.clear();
-            tmp_bindings.extend(bindings.values().copied().filter(|binding| {
-                !keep_name_symbols
-                    .as_ref()
-                    .is_some_and(|keep_name_symbols| keep_name_symbols.has_bit(binding.index()))
-            }));
+            tmp_bindings.extend(
+                bindings
+                    .values()
+                    .copied()
+                    .filter(|id| !annex_b_skip.has_bit(id.index()))
+                    .chain(
+                        annex_b_hoisted
+                            .get(&scope_id.index())
+                            .into_iter()
+                            .flat_map(|v| v.iter().copied()),
+                    )
+                    .filter(|binding| {
+                        !keep_name_symbols.as_ref().is_some_and(|keep_name_symbols| {
+                            keep_name_symbols.has_bit(binding.index())
+                        })
+                    }),
+            );
             if tmp_bindings.is_empty() {
                 continue;
             }
diff --git a/crates/oxc_minifier/tests/mangler/mod.rs b/crates/oxc_minifier/tests/mangler/mod.rs
index a0f2d0acb8d9e..ae609bc40405f 100644
--- a/crates/oxc_minifier/tests/mangler/mod.rs
+++ b/crates/oxc_minifier/tests/mangler/mod.rs
@@ -6,9 +6,12 @@ use oxc_mangler::{MangleOptions, MangleOptionsKeepNames, Mangler};
 use oxc_parser::Parser;
 use oxc_span::SourceType;
 
-fn mangle(source_text: &str, options: MangleOptions) -> String {
+fn mangle_with_source_type(
+    source_text: &str,
+    options: MangleOptions,
+    source_type: SourceType,
+) -> String {
     let allocator = Allocator::default();
-    let source_type = SourceType::mjs().with_unambiguous(true);
     let ret = Parser::new(&allocator, source_text, source_type).parse();
     assert!(ret.errors.is_empty(), "Parser errors: {:?}", ret.errors);
     let program = ret.program;
@@ -20,6 +23,14 @@ fn mangle(source_text: &str, options: MangleOptions) -> String {
         .code
 }
 
+fn mangle(source_text: &str, options: MangleOptions) -> String {
+    mangle_with_source_type(source_text, options, SourceType::mjs().with_unambiguous(true))
+}
+
+fn mangle_script(source_text: &str, options: MangleOptions) -> String {
+    mangle_with_source_type(source_text, options, SourceType::script())
+}
+
 fn test(source_text: &str, expected: &str, options: MangleOptions) {
     let mangled = mangle(source_text, options);
     let expected = {
@@ -207,3 +218,50 @@ fn private_member_mangling() {
         insta::assert_snapshot!("private_member_mangling", snapshot);
     });
 }
+
+/// In sloppy mode (script), function declarations inside blocks have Annex B.3.2.1 semantics:
+/// they create an implicit `var`-like assignment in the enclosing function scope at block exit.
+/// The mangler must not assign the same name to such a function and an outer `var` binding,
+/// or the Annex B assignment would overwrite the outer variable.
+#[test]
+fn annex_b_block_scoped_function() {
+    let cases = [
+        // Core bug: var + block function in if statement (vitejs/vite#22009)
+        "function _() { var x = 1; if (true) { function y() {} } use(x); }",
+        // var + block function in try block (oxc-project/oxc#14316)
+        "function _() { var x = 1; try { function y() {} } finally {} use(x); }",
+        // var + block function in plain block
+        "function _() { var x = 1; { function y() {} } use(x); }",
+        // Parameter + block function (Annex B doesn't overwrite params, but still safe)
+        "function _(x) { if (true) { function y() {} } use(x); }",
+        // Deeply nested blocks
+        "function _() { var x = 1; { { if (true) { function y() {} } } } use(x); }",
+        // Arrow callback + block function (oxc-project/oxc#20610)
+        "function outer(input) { let data = input; Object.keys(data).forEach((key) => { let value = data[key]; if (value) { function appendStyle() {} console.log(appendStyle); } console.log(value); }); }",
+        // Multiple block functions in same scope
+        "function _() { var x = 1; if (true) { function y() {} function z() {} } use(x); }",
+        // Block function referencing outer var (liveness already covers this, but verify)
+        "function _() { var x = 1; if (true) { function y() { return x; } } use(x); }",
+        // Sibling block with let reusing function's slot is safe (let is truly block-scoped)
+        "function _() { var x = 1; if (true) { function y() {} } { let z = 2; use(z); } use(x); }",
+        // Sibling Annex B functions both get fresh slots
+        "function _() { var x = 1; if (true) { function y() {} } if (true) { function z() {} } use(x); }",
+        // Catch parameter in sibling scope
+        "function _() { var x = 1; if (true) { function y() {} } try { throw 0; } catch (z) { use(z); } use(x); }",
+        // Sibling var is hoisted to function scope, so no conflict
+        "function _() { var x = 1; if (true) { function y() {} } { var z = 2; use(z); } use(x); }",
+        // Annex B function reuses name from sibling function scope (hoisting enables this)
+        "function _() { function foo() { var x; use(x); } function bar() { if (true) { function baz() {} use(baz); } } }",
+    ];
+
+    let mut snapshot = String::new();
+    cases.into_iter().fold(&mut snapshot, |w, case| {
+        let options = MangleOptions::default();
+        write!(w, "{case}\n{}\n", mangle_script(case, options)).unwrap();
+        w
+    });
+
+    insta::with_settings!({ prepend_module_to_snapshot => false, omit_expression => true }, {
+        insta::assert_snapshot!("annex_b_block_scoped_function", snapshot);
+    });
+}
diff --git a/crates/oxc_minifier/tests/mangler/snapshots/annex_b_block_scoped_function.snap b/crates/oxc_minifier/tests/mangler/snapshots/annex_b_block_scoped_function.snap
new file mode 100644
index 0000000000000..b7a63a67c79f8
--- /dev/null
+++ b/crates/oxc_minifier/tests/mangler/snapshots/annex_b_block_scoped_function.snap
@@ -0,0 +1,150 @@
+---
+source: crates/oxc_minifier/tests/mangler/mod.rs
+---
+function _() { var x = 1; if (true) { function y() {} } use(x); }
+function _() {
+	var e = 1;
+	if (true) {
+		function t() {}
+	}
+	use(e);
+}
+
+function _() { var x = 1; try { function y() {} } finally {} use(x); }
+function _() {
+	var e = 1;
+	try {
+		function t() {}
+	} finally {}
+	use(e);
+}
+
+function _() { var x = 1; { function y() {} } use(x); }
+function _() {
+	var e = 1;
+	{
+		function t() {}
+	}
+	use(e);
+}
+
+function _(x) { if (true) { function y() {} } use(x); }
+function _(e) {
+	if (true) {
+		function t() {}
+	}
+	use(e);
+}
+
+function _() { var x = 1; { { if (true) { function y() {} } } } use(x); }
+function _() {
+	var e = 1;
+	{
+		{
+			if (true) {
+				function t() {}
+			}
+		}
+	}
+	use(e);
+}
+
+function outer(input) { let data = input; Object.keys(data).forEach((key) => { let value = data[key]; if (value) { function appendStyle() {} console.log(appendStyle); } console.log(value); }); }
+function outer(e) {
+	let t = e;
+	Object.keys(t).forEach((e) => {
+		let n = t[e];
+		if (n) {
+			function r() {}
+			console.log(r);
+		}
+		console.log(n);
+	});
+}
+
+function _() { var x = 1; if (true) { function y() {} function z() {} } use(x); }
+function _() {
+	var e = 1;
+	if (true) {
+		function t() {}
+		function n() {}
+	}
+	use(e);
+}
+
+function _() { var x = 1; if (true) { function y() { return x; } } use(x); }
+function _() {
+	var e = 1;
+	if (true) {
+		function t() {
+			return e;
+		}
+	}
+	use(e);
+}
+
+function _() { var x = 1; if (true) { function y() {} } { let z = 2; use(z); } use(x); }
+function _() {
+	var e = 1;
+	if (true) {
+		function t() {}
+	}
+	{
+		let e = 2;
+		use(e);
+	}
+	use(e);
+}
+
+function _() { var x = 1; if (true) { function y() {} } if (true) { function z() {} } use(x); }
+function _() {
+	var e = 1;
+	if (true) {
+		function t() {}
+	}
+	if (true) {
+		function n() {}
+	}
+	use(e);
+}
+
+function _() { var x = 1; if (true) { function y() {} } try { throw 0; } catch (z) { use(z); } use(x); }
+function _() {
+	var e = 1;
+	if (true) {
+		function t() {}
+	}
+	try {
+		throw 0;
+	} catch (e) {
+		use(e);
+	}
+	use(e);
+}
+
+function _() { var x = 1; if (true) { function y() {} } { var z = 2; use(z); } use(x); }
+function _() {
+	var e = 1;
+	if (true) {
+		function t() {}
+	}
+	{
+		var n = 2;
+		use(n);
+	}
+	use(e);
+}
+
+function _() { function foo() { var x; use(x); } function bar() { if (true) { function baz() {} use(baz); } } }
+function _() {
+	function e() {
+		var e;
+		use(e);
+	}
+	function t() {
+		if (true) {
+			function e() {}
+			use(e);
+		}
+	}
+}