diff --git a/preprocess.c b/preprocess.c
index 5cb740028e1998b66ef9b0be1bd98d8504a0f514..82a0ef383333f46c6c964944f47c116869bae9f6 100644
--- a/preprocess.c
+++ b/preprocess.c
@@ -5,6 +5,7 @@ struct Macro {
   Macro *next;
   char *name;
   Token *body;
+  bool deleted;
 };
 
 // `#if` can be nested, so we use a stack to manage nested `#if`s.
@@ -150,7 +151,7 @@ static Macro *find_macro(Token *tok) {
 
   for (Macro *m = macros; m; m = m->next)
     if (strlen(m->name) == tok->len && !strncmp(m->name, tok->loc, tok->len))
-      return m;
+      return m->deleted ? NULL : m;
   return NULL;
 }
 
@@ -223,6 +224,18 @@ static Token *preprocess2(Token *tok) {
       continue;
     }
 
+    if (equal(tok, "undef")) {
+      tok = tok->next;
+      if (tok->kind != TK_IDENT)
+        error_tok(tok, "macro name must be an identifier");
+      char *name = strndup(tok->loc, tok->len);
+      tok = skip_line(tok->next);
+
+      Macro *m = add_macro(name, NULL);
+      m->deleted = true;
+      continue;
+    }
+
     if (equal(tok, "if")) {
       long val = eval_const_expr(&tok, tok);
       push_cond_incl(start, val);
diff --git a/test/macro.c b/test/macro.c
index c26008087a994ba722e8f1f2223f902c2b07d010..6481464a2438eace6bc5138f4b2598a20231d2bf 100644
--- a/test/macro.c
+++ b/test/macro.c
@@ -121,6 +121,13 @@ int main() {
 #define END )
   ASSERT_ 5, if, five END;
 
+#undef ASSERT_
+#undef if
+#undef five
+#undef END
+
+  if (0);
+
   printf("OK\n");
   return 0;
 }