diff --git a/src/validators/union.rs b/src/validators/union.rs
index bfe744212..531e2ca9d 100644
--- a/src/validators/union.rs
+++ b/src/validators/union.rs
@@ -113,7 +113,7 @@ impl UnionValidator {
         let strict = state.strict_or(self.strict);
         let mut errors = MaybeErrors::new(self.custom_error.as_ref());
 
-        let mut best_match: Option<(Py<PyAny>, Exactness, Option<usize>)> = None;
+        let mut best_match: Option<(Py<PyAny>, Exactness, Option<usize>, usize, f64)> = None;
 
         for (choice, label) in &self.choices {
             let state = &mut state.rebind_extra(|extra| {
@@ -141,25 +141,24 @@ impl UnionValidator {
 
                         let new_exactness = state.exactness.unwrap_or(Exactness::Lax);
                         let new_fields_set_count = state.fields_set_count;
+                        let new_fields_set_percentage = new_fields_set_count.map(|count| {
+                            let total_fields = input.as_dict().map_or(0, |dict| dict.len());
+                            count as f64 / total_fields as f64
+                        });
 
-                        // we use both the exactness and the fields_set_count to determine the best union member match
-                        // if fields_set_count is available for the current best match and the new candidate, we use this
-                        // as the primary metric. If the new fields_set_count is greater, the new candidate is better.
-                        // if the fields_set_count is the same, we use the exactness as a tie breaker to determine the best match.
-                        // if the fields_set_count is not available for either the current best match or the new candidate,
-                        // we use the exactness to determine the best match.
                         let new_success_is_best_match: bool =
                             best_match
                                 .as_ref()
-                                .map_or(true, |(_, cur_exactness, cur_fields_set_count)| {
-                                    match (*cur_fields_set_count, new_fields_set_count) {
+                                .map_or(true, |(_, cur_exactness, cur_fields_set_count, cur_fields_set_count_value, cur_fields_set_percentage)| {
+                                    match (cur_fields_set_count, new_fields_set_count) {
                                         (Some(cur), Some(new)) if cur != new => cur < new,
+                                        (Some(_), Some(_)) => cur_fields_set_percentage < new_fields_set_percentage,
                                         _ => *cur_exactness < new_exactness,
                                     }
                                 });
 
                         if new_success_is_best_match {
-                            best_match = Some((new_success, new_exactness, new_fields_set_count));
+                            best_match = Some((new_success, new_exactness, new_fields_set_count, new_fields_set_count.unwrap_or(0), new_fields_set_percentage.unwrap_or(0.0)));
                         }
                     }
                 },
@@ -177,7 +176,7 @@ impl UnionValidator {
         state.exactness = old_exactness;
         state.fields_set_count = old_fields_set_count;
 
-        if let Some((best_match, exactness, fields_set_count)) = best_match {
+        if let Some((best_match, exactness, fields_set_count, _, _)) = best_match {
             state.floor_exactness(exactness);
             if let Some(count) = fields_set_count {
                 state.add_fields_set(count);
diff --git a/tests/validators/test_union.py b/tests/validators/test_union.py
index 1951fb3de..009317760 100644
--- a/tests/validators/test_union.py
+++ b/tests/validators/test_union.py
@@ -1333,3 +1333,45 @@ class Model:
 
     assert isinstance(validator.validate_python({'x': {'foo': 'foo'}}).x, Foo)
     assert isinstance(validator.validate_python({'x': {'bar': 'bar'}}).x, Bar)
+
+
+class A:
+    a: int
+
+
+class B(A):
+    b: int
+
+
+def test_discriminated_union():
+    v = SchemaValidator(
+        {
+            'type': 'union',
+            'choices': [
+                {
+                    'type': 'model',
+                    'cls': A,
+                    'schema': {
+                        'type': 'model-fields',
+                        'fields': {
+                            'a': {'type': 'model-field', 'schema': {'type': 'int'}},
+                        },
+                    },
+                },
+                {
+                    'type': 'model',
+                    'cls': B,
+                    'schema': {
+                        'type': 'model-fields',
+                        'fields': {
+                            'a': {'type': 'model-field', 'schema': {'type': 'int'}},
+                            'b': {'type': 'model-field', 'schema': {'type': 'int'}},
+                        },
+                    },
+                },
+            ],
+        }
+    )
+
+    assert isinstance(v.validate_python({'a': 1}), A)
+    assert isinstance(v.validate_python({'a': 1, 'b': 2}), B)