diff --git a/src/core/inttypes.c b/src/core/inttypes.c
index d985c5c7..cc7a5787 100644
--- a/src/core/inttypes.c
+++ b/src/core/inttypes.c
@@ -239,13 +239,15 @@ JANET_CORE_FN(cfun_to_number,
 }
 
 JANET_CORE_FN(cfun_to_bytes,
-              "(int/to-bytes value &opt endianness)",
-              "Convert an `int/s64` or `int/u64` into an 8-element tuple of bytes.\n"
-              "the endianness paramater indicates the byte order:\n"
+              "(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, 2);
+    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]);
     }
@@ -266,13 +268,29 @@ JANET_CORE_FN(cfun_to_bytes,
         }
     }
 
-    uint8_t *bytes = janet_unwrap_abstract(argv[0]);
-    Janet *bytes_tuple = janet_tuple_begin(8);
-    for (int i = 0; i < 8; ++i) {
-        bytes_tuple[i] = janet_wrap_integer(bytes[reverse ? 7 - i : i]);
+    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);
     }
 
-    return janet_wrap_tuple(janet_tuple_end(bytes_tuple));
+    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);
 }
 
 /*
diff --git a/test/suite0006.janet b/test/suite0006.janet
index 52309c01..34633ebd 100644
--- a/test/suite0006.janet
+++ b/test/suite0006.janet
@@ -92,17 +92,25 @@
  (:/ (int/s64 "-0x8000_0000_0000_0000") -1))
 
 # int/s64 and int/u64 serialization
-(assert (= (int/to-bytes (u64 0)) [0 0 0 0 0 0 0 0]))
+(assert (deep= (int/to-bytes (u64 0)) @"\x00\x00\x00\x00\x00\x00\x00\x00"))
 
-(assert (= (int/to-bytes (i64 1) :le) [1 0 0 0 0 0 0 0]))
-(assert (= (int/to-bytes (i64 1) :be) [0 0 0 0 0 0 0 1]))
-(assert (= (int/to-bytes (i64 -1)) [0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF]))
-(assert (= (int/to-bytes (i64 -5) :be) [0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF (- 0xFF 4)]))
+(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 (= (int/to-bytes (u64 1) :le) [1 0 0 0 0 0 0 0]))
-(assert (= (int/to-bytes (u64 1) :be) [0 0 0 0 0 0 0 1]))
-(assert (= (int/to-bytes (u64 300) :be) [0 0 0 0 0 0 0x01 0x2C]))
+(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))
@@ -111,6 +119,10 @@
   "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)