diff --git a/vlib/math/unsigned/uint128.v b/vlib/math/unsigned/uint128.v
index a22c0d5100..3195713d8a 100644
--- a/vlib/math/unsigned/uint128.v
+++ b/vlib/math/unsigned/uint128.v
@@ -423,12 +423,20 @@ pub fn uint128_new(lo u64, hi u64) Uint128 {
 	return Uint128{lo, hi}
 }
 
-// unint_from_dec_str returns an error or new Uint128 from given string
+// unint_from_dec_str returns an error or new Uint128 from given string.
+// The `_` character is allowed as a separator.
 pub fn uint128_from_dec_str(value string) !Uint128 {
 	mut res := unsigned.uint128_zero
-	for b_ in value.bytes() {
-		b := b_ - '0'.bytes()[0]
+	underscore := `_`
+
+	for b_ in value {
+		b := b_ - `0`
 		if b > 9 {
+			// allow _ as a separator in decimal strings
+			if b_ == underscore {
+				continue
+			}
+
 			return error('invalid character "${b}"')
 		}
 
diff --git a/vlib/math/unsigned/uint128_test.v b/vlib/math/unsigned/uint128_test.v
index 5ad59a9eb4..658db896fd 100644
--- a/vlib/math/unsigned/uint128_test.v
+++ b/vlib/math/unsigned/uint128_test.v
@@ -97,3 +97,76 @@ fn test_leading_zeros() {
 		assert zeros == tc.zeros
 	}
 }
+
+fn test_separators() {
+	// numbers of varying lengths and a random
+	// scattering of '_' throughout.
+	test_strings := [
+		'_',
+		'__',
+		'_0',
+		'0_',
+		'_0_',
+		'_1',
+		'1_',
+		'_1_',
+		'1_2',
+		'_12',
+		'12_',
+		'12_3',
+		'1_23_4',
+		'12_345',
+		'_123_456_',
+		'1_234_567',
+		'1234_5678',
+		'_123456789',
+		'1234567890_',
+		'0_123_456_789_0',
+		'90_12_345_67890',
+		'8901_234_567890_',
+		'_7890_123456789_0',
+		'678901234_567890',
+		'567890_1234567890',
+		'4567890123__4567890',
+		'_34567890123_4567890',
+		'2345678_90123_4567890',
+		'123456789_01_234567890',
+		'01234567_8901234567890',
+		'9012345678901_234567890__',
+		'8_90123456_78901234567890',
+		'78901234567890_123_4567890',
+		'___67890123_4_5_6_78901234567890___',
+		'567890123_45678901234567890',
+		'45_67890123456789_01234567890',
+		'3456789012_345678901234567890',
+		'234_567890_1234_5__67890_1234567890_',
+		'12345678_90123_456789_0123_4567890',
+		'0_123_456_789_012_345_678_901_234_567_890',
+		'90_123456__78901__234567_8901_234567890',
+		'890123456789012345678901234567_890',
+		'7890_1234567_8901234567890_1234567890',
+		'67890123_45678_901234567_8901234567890',
+		'567890_1234567890_12345678901_234567890',
+		'45678_9012345_6789012345678901234567890_',
+		'34567890123456_789012345678901234567890',
+		'234567_890_1234567890_1234567890_1234567890',
+		'334567890_1234567890_1234567890_1234567890',
+		'340282360_0000000000_0000000000_0000000000',
+		'340282366_9209384634_2648111928_4349108225',
+		'340282366_9209384634_6337460743_1768211455',
+	]
+
+	for ts in test_strings {
+		with := unsigned.uint128_from_dec_str(ts) or {
+			assert false, 'invalid Uint128 string ${ts}'
+			panic('')
+		}
+
+		without := unsigned.uint128_from_dec_str(ts.replace('_', '')) or {
+			assert false, 'invalid Uint128 string ${ts.replace('_', '')}'
+			panic('')
+		}
+
+		assert with == without
+	}
+}