diff --git a/CHANGELOG.md b/CHANGELOG.md
index bcd4dfd..b8d4dc8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 0.5.1 (unreleased)
+
+- Added `angular_distance` function
+
 ## 0.5.0 (2023-08-28)
 
 - Added HNSW index type
diff --git a/sql/vector--0.5.0--0.5.1.sql b/sql/vector--0.5.0--0.5.1.sql
new file mode 100644
index 0000000..a9d72fe
--- /dev/null
+++ b/sql/vector--0.5.0--0.5.1.sql
@@ -0,0 +1,5 @@
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION vector UPDATE TO '0.5.1'" to load this file. \quit
+
+CREATE FUNCTION angular_distance(vector, vector) RETURNS float8
+	AS 'MODULE_PATHNAME' LANGUAGE C IMMUTABLE STRICT PARALLEL SAFE;
diff --git a/sql/vector.sql b/sql/vector.sql
index 137931f..69ec142 100644
--- a/sql/vector.sql
+++ b/sql/vector.sql
@@ -87,6 +87,9 @@ CREATE FUNCTION vector_l2_squared_distance(vector, vector) RETURNS float8
 CREATE FUNCTION vector_negative_inner_product(vector, vector) RETURNS float8
 	AS 'MODULE_PATHNAME' LANGUAGE C IMMUTABLE STRICT PARALLEL SAFE;
 
+CREATE FUNCTION angular_distance(vector, vector) RETURNS float8
+	AS 'MODULE_PATHNAME' LANGUAGE C IMMUTABLE STRICT PARALLEL SAFE;
+
 CREATE FUNCTION vector_spherical_distance(vector, vector) RETURNS float8
 	AS 'MODULE_PATHNAME' LANGUAGE C IMMUTABLE STRICT PARALLEL SAFE;
 
diff --git a/src/vector.c b/src/vector.c
index d3ebedb..99ef54d 100644
--- a/src/vector.c
+++ b/src/vector.c
@@ -684,6 +684,49 @@ cosine_distance(PG_FUNCTION_ARGS)
 	PG_RETURN_FLOAT8(1.0 - similarity);
 }
 
+/*
+ * Get the angular distance between two vectors
+ */
+PGDLLEXPORT PG_FUNCTION_INFO_V1(angular_distance);
+Datum
+angular_distance(PG_FUNCTION_ARGS)
+{
+	Vector	   *a = PG_GETARG_VECTOR_P(0);
+	Vector	   *b = PG_GETARG_VECTOR_P(1);
+	float	   *ax = a->x;
+	float	   *bx = b->x;
+	float		distance = 0.0;
+	float		norma = 0.0;
+	float		normb = 0.0;
+	double		similarity;
+
+	CheckDims(a, b);
+
+	/* Auto-vectorized */
+	for (int i = 0; i < a->dim; i++)
+	{
+		distance += ax[i] * bx[i];
+		norma += ax[i] * ax[i];
+		normb += bx[i] * bx[i];
+	}
+
+	similarity = (double) distance / sqrt((double) norma * (double) normb);
+
+#ifdef _MSC_VER
+	/* /fp:fast may not propagate NaN */
+	if (isnan(similarity))
+		PG_RETURN_FLOAT8(NAN);
+#endif
+
+	/* Prevent NaN with acos with loss of precision */
+	if (similarity > 1)
+		similarity = 1;
+	else if (similarity < -1)
+		similarity = -1;
+
+	PG_RETURN_FLOAT8(acos(similarity) / M_PI);
+}
+
 /*
  * Get the distance for spherical k-means
  * Currently uses angular distance since needs to satisfy triangle inequality
diff --git a/test/expected/functions.out b/test/expected/functions.out
index 16092ba..648e80c 100644
--- a/test/expected/functions.out
+++ b/test/expected/functions.out
@@ -152,6 +152,50 @@ SELECT l1_distance('[3e38]', '[-3e38]');
     Infinity
 (1 row)
 
+SELECT angular_distance('[1,2]', '[2,4]');
+ angular_distance 
+------------------
+                0
+(1 row)
+
+SELECT angular_distance('[1,2]', '[0,0]');
+ angular_distance 
+------------------
+              NaN
+(1 row)
+
+SELECT angular_distance('[1,1]', '[1,1]');
+ angular_distance 
+------------------
+                0
+(1 row)
+
+SELECT angular_distance('[1,1]', '[-1,-1]');
+ angular_distance 
+------------------
+                1
+(1 row)
+
+SELECT angular_distance('[1,2]', '[3]');
+ERROR:  different vector dimensions 2 and 1
+SELECT angular_distance('[1,1]', '[1.1,1.1]');
+ angular_distance 
+------------------
+                0
+(1 row)
+
+SELECT angular_distance('[1,1]', '[-1.1,-1.1]');
+ angular_distance 
+------------------
+                1
+(1 row)
+
+SELECT angular_distance('[3e38]', '[3e38]');
+ angular_distance 
+------------------
+              NaN
+(1 row)
+
 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 78ec8c1..b70f87b 100644
--- a/test/sql/functions.sql
+++ b/test/sql/functions.sql
@@ -36,6 +36,15 @@ SELECT l1_distance('[0,0]', '[0,1]');
 SELECT l1_distance('[1,2]', '[3]');
 SELECT l1_distance('[3e38]', '[-3e38]');
 
+SELECT angular_distance('[1,2]', '[2,4]');
+SELECT angular_distance('[1,2]', '[0,0]');
+SELECT angular_distance('[1,1]', '[1,1]');
+SELECT angular_distance('[1,1]', '[-1,-1]');
+SELECT angular_distance('[1,2]', '[3]');
+SELECT angular_distance('[1,1]', '[1.1,1.1]');
+SELECT angular_distance('[1,1]', '[-1.1,-1.1]');
+SELECT angular_distance('[3e38]', '[3e38]');
+
 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;