diff --git a/src/validators/config.rs b/src/validators/config.rs
index 8ffcfd0f5..a43a7643a 100644
--- a/src/validators/config.rs
+++ b/src/validators/config.rs
@@ -1,7 +1,8 @@
 use std::borrow::Cow;
 use std::str::FromStr;
 
-use base64::Engine;
+use base64::engine::general_purpose::{STANDARD, URL_SAFE};
+use base64::{DecodeError, Engine};
 use pyo3::types::{PyDict, PyString};
 use pyo3::{intern, prelude::*};
 
@@ -28,14 +29,18 @@ impl ValBytesMode {
     pub fn deserialize_string<'py>(self, s: &str) -> Result<EitherBytes<'_, 'py>, ErrorType> {
         match self.ser {
             BytesMode::Utf8 => Ok(EitherBytes::Cow(Cow::Borrowed(s.as_bytes()))),
-            BytesMode::Base64 => match base64::engine::general_purpose::URL_SAFE.decode(s) {
-                Ok(bytes) => Ok(EitherBytes::from(bytes)),
-                Err(err) => Err(ErrorType::BytesInvalidEncoding {
+            BytesMode::Base64 => URL_SAFE
+                .decode(s)
+                .or_else(|err| match err {
+                    DecodeError::InvalidByte(_, b'/' | b'+') => STANDARD.decode(s),
+                    _ => Err(err),
+                })
+                .map(EitherBytes::from)
+                .map_err(|err| ErrorType::BytesInvalidEncoding {
                     encoding: "base64".to_string(),
                     encoding_error: err.to_string(),
                     context: None,
                 }),
-            },
             BytesMode::Hex => match hex::decode(s) {
                 Ok(vec) => Ok(EitherBytes::from(vec)),
                 Err(err) => Err(ErrorType::BytesInvalidEncoding {
diff --git a/tests/test_json.py b/tests/test_json.py
index 03855ae67..f22850704 100644
--- a/tests/test_json.py
+++ b/tests/test_json.py
@@ -379,19 +379,26 @@ def test_partial_parse():
 
 
 def test_json_bytes_base64_round_trip():
-    data = b'hello'
-    encoded = b'"aGVsbG8="'
-    assert to_json(data, bytes_mode='base64') == encoded
+    data = b'\xd8\x07\xc1Tx$\x91F%\xf3\xf3I\xca\xd8@\x0c\xee\xc3\xab\xff\x7f\xd3\xcd\xcd\xf9\xc2\x10\xe4\xa1\xb01e'
+    encoded_std = b'"2AfBVHgkkUYl8/NJythADO7Dq/9/083N+cIQ5KGwMWU="'
+    encoded_url = b'"2AfBVHgkkUYl8_NJythADO7Dq_9_083N-cIQ5KGwMWU="'
+    assert to_json(data, bytes_mode='base64') == encoded_url
 
     v = SchemaValidator({'type': 'bytes'}, {'val_json_bytes': 'base64'})
-    assert v.validate_json(encoded) == data
+    assert v.validate_json(encoded_url) == data
+    assert v.validate_json(encoded_std) == data
+
+    with pytest.raises(ValidationError) as exc:
+        v.validate_json('"wrong!"')
+    [details] = exc.value.errors()
+    assert details['type'] == 'bytes_invalid_encoding'
 
-    assert to_json({'key': data}, bytes_mode='base64') == b'{"key":"aGVsbG8="}'
+    assert to_json({'key': data}, bytes_mode='base64') == b'{"key":' + encoded_url + b'}'
     v = SchemaValidator(
         {'type': 'dict', 'keys_schema': {'type': 'str'}, 'values_schema': {'type': 'bytes'}},
         {'val_json_bytes': 'base64'},
     )
-    assert v.validate_json('{"key":"aGVsbG8="}') == {'key': data}
+    assert v.validate_json(b'{"key":' + encoded_url + b'}') == {'key': data}
 
 
 def test_json_bytes_base64_invalid():