diff --git a/src/core/inttypes.c b/src/core/inttypes.c
index 5de40112..cc7a5787 100644
--- a/src/core/inttypes.c
+++ b/src/core/inttypes.c
@@ -238,6 +238,61 @@ JANET_CORE_FN(cfun_to_number,
     janet_panicf("expected int/u64 or int/s64, got %q", argv[0]);
 }
 
+JANET_CORE_FN(cfun_to_bytes,
+              "(int/to-bytes value &opt endianness buffer)",
+              "Write the bytes of an `int/s64` or `int/u64` into a buffer.\n"
+              "The `buffer` parameter specifies an existing buffer to write to, if unset a new buffer will be created.\n"
+              "Returns the modified buffer.\n"
+              "The `endianness` paramater indicates the byte order:\n"
+              "- `nil` (unset): system byte order\n"
+              "- `:le`: little-endian, least significant byte first\n"
+              "- `:be`: big-endian, most significant byte first\n") {
+    janet_arity(argc, 1, 3);
+    if (janet_is_int(argv[0]) == JANET_INT_NONE) {
+        janet_panicf("int/to-bytes: expected an int/s64 or int/u64, got %q", argv[0]);
+    }
+
+    int reverse = 0;
+    if (argc > 1 && !janet_checktype(argv[1], JANET_NIL)) {
+        JanetKeyword endianness_kw = janet_getkeyword(argv, 1);
+        if (!janet_cstrcmp(endianness_kw, "le")) {
+#if JANET_BIG_ENDIAN
+            reverse = 1;
+#endif
+        } else if (!janet_cstrcmp(endianness_kw, "be")) {
+#if JANET_LITTLE_ENDIAN
+            reverse = 1;
+#endif
+        } else {
+            janet_panicf("int/to-bytes: expected endianness :le, :be or nil, got %v", argv[1]);
+        }
+    }
+
+    JanetBuffer *buffer = NULL;
+    if (argc > 2 && !janet_checktype(argv[2], JANET_NIL)) {
+        if (!janet_checktype(argv[2], JANET_BUFFER)) {
+            janet_panicf("int/to-bytes: expected buffer or nil, got %q", argv[2]);
+        }
+
+        buffer = janet_unwrap_buffer(argv[2]);
+        janet_buffer_extra(buffer, 8);
+    } else {
+        buffer = janet_buffer(8);
+    }
+
+    uint8_t *bytes = janet_unwrap_abstract(argv[0]);
+    if (reverse) {
+        for (int i = 0; i < 8; ++i) {
+            buffer->data[buffer->count + 7 - i] = bytes[i];
+        }
+    } else {
+        memcpy(buffer->data + buffer->count, bytes, 8);
+    }
+    buffer->count += 8;
+
+    return janet_wrap_buffer(buffer);
+}
+
 /*
  * Code to support polymorphic comparison.
  * int/u64 and int/s64 support a "compare" method that allows
@@ -546,6 +601,7 @@ void janet_lib_inttypes(JanetTable *env) {
         JANET_CORE_REG("int/s64", cfun_it_s64_new),
         JANET_CORE_REG("int/u64", cfun_it_u64_new),
         JANET_CORE_REG("int/to-number", cfun_to_number),
+        JANET_CORE_REG("int/to-bytes", cfun_to_bytes),
         JANET_REG_END
     };
     janet_core_cfuns_ext(env, NULL, it_cfuns);
diff --git a/test/suite0006.janet b/test/suite0006.janet
index 3c4cd551..34633ebd 100644
--- a/test/suite0006.janet
+++ b/test/suite0006.janet
@@ -91,6 +91,39 @@
  "trap INT64_MIN / -1"
  (:/ (int/s64 "-0x8000_0000_0000_0000") -1))
 
+# int/s64 and int/u64 serialization
+(assert (deep= (int/to-bytes (u64 0)) @"\x00\x00\x00\x00\x00\x00\x00\x00"))
+
+(assert (deep= (int/to-bytes (i64 1) :le) @"\x01\x00\x00\x00\x00\x00\x00\x00"))
+(assert (deep= (int/to-bytes (i64 1) :be) @"\x00\x00\x00\x00\x00\x00\x00\x01"))
+(assert (deep= (int/to-bytes (i64 -1)) @"\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFF"))
+(assert (deep= (int/to-bytes (i64 -5) :be) @"\xFF\xFF\xFF\xFF\xFF\xFF\xFF\xFB"))
+
+(assert (deep= (int/to-bytes (u64 1) :le) @"\x01\x00\x00\x00\x00\x00\x00\x00"))
+(assert (deep= (int/to-bytes (u64 1) :be) @"\x00\x00\x00\x00\x00\x00\x00\x01"))
+(assert (deep= (int/to-bytes (u64 300) :be) @"\x00\x00\x00\x00\x00\x00\x01\x2C"))
+
+# int/s64 int/u64 to existing buffer
+(let [buf1 @""
+      buf2 @"abcd"]
+  (assert (deep= (int/to-bytes (i64 1) :le buf1) @"\x01\x00\x00\x00\x00\x00\x00\x00"))
+  (assert (deep= buf1 @"\x01\x00\x00\x00\x00\x00\x00\x00"))
+  (assert (deep= (int/to-bytes (u64 300) :be buf2) @"abcd\x00\x00\x00\x00\x00\x00\x01\x2C")))
+
+# int/s64 and int/u64 paramater type checking
+(assert-error
+ "bad value passed to int/to-bytes"
+ (int/to-bytes 1))
+
+(assert-error
+  "invalid endianness passed to int/to-bytes"
+   (int/to-bytes (u64 0) :little))
+
+(assert-error
+  "invalid buffer passed to int/to-bytes"
+   (int/to-bytes (u64 0) :little :buffer))
+
+
 # Dynamic bindings
 (setdyn :a 10)
 (assert (= 40 (with-dyns [:a 25 :b 15] (+ (dyn :a) (dyn :b)))) "dyn usage 1")