diff --git a/changes/3846-chornsby.md b/changes/3846-chornsby.md
new file mode 100644
index 0000000..f8daac8
--- /dev/null
+++ b/changes/3846-chornsby.md
@@ -0,0 +1 @@
+Fix validation of discriminated union fields with an alias when passing a model instance
diff --git a/pydantic/fields.py b/pydantic/fields.py
index 10b59fd..1c319cc 100644
--- a/pydantic/fields.py
+++ b/pydantic/fields.py
@@ -1105,7 +1105,7 @@ class ModelField(Representation):
         except TypeError:
             try:
                 # BaseModel or dataclass
-                discriminator_value = getattr(v, self.discriminator_alias)
+                discriminator_value = getattr(v, self.discriminator_key)
             except (AttributeError, TypeError):
                 return v, ErrorWrapper(MissingDiscriminator(discriminator_key=self.discriminator_key), loc)
 
diff --git a/tests/test_discrimated_union.py b/tests/test_discrimated_union.py
index a4dd501..a099fca 100644
--- a/tests/test_discrimated_union.py
+++ b/tests/test_discrimated_union.py
@@ -267,6 +267,24 @@ def test_discriminated_union_basemodel_instance_value():
     assert isinstance(t, Top)
 
 
+def test_discriminated_union_basemodel_instance_value_with_alias():
+    class A(BaseModel):
+        literal: Literal['a'] = Field(alias='lit')
+
+    class B(BaseModel):
+        literal: Literal['b'] = Field(alias='lit')
+
+        class Config:
+            allow_population_by_field_name = True
+
+    class Top(BaseModel):
+        sub: Union[A, B] = Field(..., discriminator='literal')
+
+    assert Top(sub=A(lit='a')).sub.literal == 'a'
+    assert Top(sub=B(lit='b')).sub.literal == 'b'
+    assert Top(sub=B(literal='b')).sub.literal == 'b'
+
+
 def test_discriminated_union_int():
     class A(BaseModel):
         l: Literal[1]