diff --git a/chibicc.h b/chibicc.h
index ba35441f18d3a30443e7c67b4a393d8349f438b6..34784eae8ee20fa207b67107d70fdb8fceb10611 100644
--- a/chibicc.h
+++ b/chibicc.h
@@ -99,6 +99,8 @@ Token *tokenize_file(char *filename);
 // preprocess.c
 //
 
+void init_macros(void);
+void define_macro(char *name, char *buf);
 Token *preprocess(Token *tok);
 
 //
diff --git a/main.c b/main.c
index 8fcc1a368aa0551a01c390209eb147a2d1226b8f..2f920e4e440ed1685aaaf9f26dac5de3ade39c2d 100644
--- a/main.c
+++ b/main.c
@@ -40,6 +40,14 @@ static void add_default_include_paths(char *argv0) {
   strarray_push(&include_paths, "/usr/include");
 }
 
+static void define(char *str) {
+  char *eq = strchr(str, '=');
+  if (eq)
+    define_macro(strndup(str, eq - str), eq + 1);
+  else
+    define_macro(str, "1");
+}
+
 static void parse_args(int argc, char **argv) {
   // Make sure that all command line options that take an argument
   // have an argument.
@@ -92,6 +100,16 @@ static void parse_args(int argc, char **argv) {
       continue;
     }
 
+    if (!strcmp(argv[i], "-D")) {
+      define(argv[++i]);
+      continue;
+    }
+
+    if (!strncmp(argv[i], "-D", 2)) {
+      define(argv[i] + 2);
+      continue;
+    }
+
     if (!strcmp(argv[i], "-cc1-input")) {
       base_file = argv[++i];
       continue;
@@ -321,6 +339,7 @@ static void run_linker(StringArray *inputs, char *output) {
 
 int main(int argc, char **argv) {
   atexit(cleanup);
+  init_macros();
   parse_args(argc, argv);
 
   if (opt_cc1) {
diff --git a/preprocess.c b/preprocess.c
index 39d84788bb4a08bf0bf88d7c0fc9fc3197c5ce9c..d1fcba3f256ab9de81d76890e76ba3c692769132 100644
--- a/preprocess.c
+++ b/preprocess.c
@@ -831,7 +831,7 @@ static Token *preprocess2(Token *tok) {
   return head.next;
 }
 
-static void define_macro(char *name, char *buf) {
+void define_macro(char *name, char *buf) {
   Token *tok = tokenize(new_file("<built-in>", 1, buf));
   add_macro(name, true, tok);
 }
@@ -854,7 +854,7 @@ static Token *line_macro(Token *tmpl) {
   return new_num_token(tmpl->line_no, tmpl);
 }
 
-static void init_macros(void) {
+void init_macros(void) {
   // Define predefined macros
   define_macro("_LP64", "1");
   define_macro("__C99_MACRO_WITH_VA_ARGS", "1");
@@ -937,7 +937,6 @@ static void join_adjacent_string_literals(Token *tok1) {
 
 // Entry point function of the preprocessor.
 Token *preprocess(Token *tok) {
-  init_macros();
   tok = preprocess2(tok);
   if (cond_incl)
     error_tok(cond_incl->tok, "unterminated conditional directive");
diff --git a/test/driver.sh b/test/driver.sh
index 80bc0db790019fb7d230b6a53595e3fb21796d08..fc5f71fd1fbc060c5d527ed5f908a585e7018304 100755
--- a/test/driver.sh
+++ b/test/driver.sh
@@ -91,4 +91,12 @@ echo foo > $tmp/dir/i-option-test
 echo "#include \"i-option-test\"" | $chibicc -I$tmp/dir -E - | grep -q foo
 check -I
 
+# -D
+echo foo | $chibicc -Dfoo -E - | grep -q 1
+check -D
+
+# -D
+echo foo | $chibicc -Dfoo=bar -E - | grep -q bar
+check -D
+
 echo OK