diff --git a/CHANGELOG.md b/CHANGELOG.md
index 67fc031..42eb112 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@ If upgrading with Postgres < 13, see [this note](https://github.com/pgvector/pgv
 - Changed storage for vector from `extended` to `external`
 - Improved performance of HNSW
 - Added support for parallel index builds for HNSW
+- Added `hamming_distance` function
 - Added validation for GUC parameters
 - Reduced memory usage for HNSW index builds
 - Reduced WAL generation for HNSW index builds
diff --git a/sql/vector--0.5.1--0.6.0.sql b/sql/vector--0.5.1--0.6.0.sql
index 8e5af7f..9c5e6a7 100644
--- a/sql/vector--0.5.1--0.6.0.sql
+++ b/sql/vector--0.5.1--0.6.0.sql
@@ -3,3 +3,6 @@
 
 -- remove this single line for Postgres < 13
 ALTER TYPE vector SET (STORAGE = external);
+
+CREATE FUNCTION hamming_distance(bytea, bytea) RETURNS float8
+	AS 'MODULE_PATHNAME' LANGUAGE C IMMUTABLE STRICT PARALLEL SAFE;
diff --git a/sql/vector.sql b/sql/vector.sql
index 4b17faa..a535720 100644
--- a/sql/vector.sql
+++ b/sql/vector.sql
@@ -290,3 +290,8 @@ CREATE OPERATOR CLASS vector_cosine_ops
 	OPERATOR 1 <=> (vector, vector) FOR ORDER BY float_ops,
 	FUNCTION 1 vector_negative_inner_product(vector, vector),
 	FUNCTION 2 vector_norm(vector);
+
+-- bytea functions
+
+CREATE FUNCTION hamming_distance(bytea, bytea) RETURNS float8
+	AS 'MODULE_PATHNAME' LANGUAGE C IMMUTABLE STRICT PARALLEL SAFE;
diff --git a/src/vector.c b/src/vector.c
index 982b176..562373d 100644
--- a/src/vector.c
+++ b/src/vector.c
@@ -10,6 +10,7 @@
 #include "lib/stringinfo.h"
 #include "libpq/pqformat.h"
 #include "port.h"				/* for strtof() */
+#include "port/pg_bitutils.h"
 #include "utils/array.h"
 #include "utils/builtins.h"
 #include "utils/float.h"
@@ -1134,3 +1135,43 @@ vector_avg(PG_FUNCTION_ARGS)
 
 	PG_RETURN_POINTER(result);
 }
+
+/*
+ * Ensure same number of bytes
+ */
+static inline void
+CheckByteLengths(unsigned long aLen, unsigned long bLen)
+{
+	if (aLen != bLen)
+		ereport(ERROR,
+				(errcode(ERRCODE_DATA_EXCEPTION),
+				 errmsg("different byte lengths %lu and %lu", aLen, bLen)));
+}
+
+/*
+ * Get the hamming distance between two binary strings
+ */
+PGDLLEXPORT PG_FUNCTION_INFO_V1(hamming_distance);
+Datum
+hamming_distance(PG_FUNCTION_ARGS)
+{
+	bytea	   *a = PG_GETARG_BYTEA_PP(0);
+	bytea	   *b = PG_GETARG_BYTEA_PP(1);
+	char	   *ax = VARDATA_ANY(a);
+	char	   *bx = VARDATA_ANY(b);
+	unsigned long aLen = VARSIZE_ANY_EXHDR(a);
+	unsigned long bLen = VARSIZE_ANY_EXHDR(b);
+	uint64		distance = 0;
+
+	CheckByteLengths(aLen, bLen);
+
+	for (unsigned long i = 0; i < aLen; i++)
+	{
+		unsigned char diff = (unsigned char) (ax[i] ^ bx[i]);
+
+		distance += pg_number_of_ones[diff];
+	}
+
+	/* TODO Decide on return type */
+	PG_RETURN_FLOAT8((double) distance);
+}
diff --git a/test/expected/functions.out b/test/expected/functions.out
index 2840688..84da2d8 100644
--- a/test/expected/functions.out
+++ b/test/expected/functions.out
@@ -158,6 +158,32 @@ SELECT l1_distance('[3e38]', '[-3e38]');
     Infinity
 (1 row)
 
+SELECT hamming_distance('\xFF', '\xFF');
+ hamming_distance 
+------------------
+                0
+(1 row)
+
+SELECT hamming_distance('\xFF', '\xFE');
+ hamming_distance 
+------------------
+                1
+(1 row)
+
+SELECT hamming_distance('\xFF', '\xFC');
+ hamming_distance 
+------------------
+                2
+(1 row)
+
+SELECT hamming_distance('\xFF', '\x00');
+ hamming_distance 
+------------------
+                8
+(1 row)
+
+SELECT hamming_distance('\xFF', '\x0000');
+ERROR:  different byte lengths 1 and 2
 SELECT avg(v) FROM unnest(ARRAY['[1,2,3]'::vector, '[3,5,7]']) v;
     avg    
 -----------
diff --git a/test/sql/functions.sql b/test/sql/functions.sql
index 914df36..fc657e2 100644
--- a/test/sql/functions.sql
+++ b/test/sql/functions.sql
@@ -37,6 +37,12 @@ SELECT l1_distance('[0,0]', '[0,1]');
 SELECT l1_distance('[1,2]', '[3]');
 SELECT l1_distance('[3e38]', '[-3e38]');
 
+SELECT hamming_distance('\xFF', '\xFF');
+SELECT hamming_distance('\xFF', '\xFE');
+SELECT hamming_distance('\xFF', '\xFC');
+SELECT hamming_distance('\xFF', '\x00');
+SELECT hamming_distance('\xFF', '\x0000');
+
 SELECT avg(v) FROM unnest(ARRAY['[1,2,3]'::vector, '[3,5,7]']) v;
 SELECT avg(v) FROM unnest(ARRAY['[1,2,3]'::vector, '[3,5,7]', NULL]) v;
 SELECT avg(v) FROM unnest(ARRAY[]::vector[]) v;