diff --git a/sqlmodel/main.py b/sqlmodel/main.py
index a5ce8fa..d343c69 100644
--- a/sqlmodel/main.py
+++ b/sqlmodel/main.py
@@ -423,11 +423,13 @@ def get_column_from_field(field: ModelField) -> Column:  # type: ignore
     index = getattr(field.field_info, "index", Undefined)
     if index is Undefined:
         index = False
+    nullable = not primary_key and _is_field_noneable(field)
+    # Override derived nullability if the nullable property is set explicitly
+    # on the field
     if hasattr(field.field_info, "nullable"):
         field_nullable = getattr(field.field_info, "nullable")
         if field_nullable != Undefined:
             nullable = field_nullable
-    nullable = not primary_key and _is_field_nullable(field)
     args = []
     foreign_key = getattr(field.field_info, "foreign_key", None)
     unique = getattr(field.field_info, "unique", False)
@@ -644,11 +646,10 @@ class SQLModel(BaseModel, metaclass=SQLModelMetaclass, registry=default_registry
         return cls.__name__.lower()
 
 
-def _is_field_nullable(field: ModelField) -> bool:
+def _is_field_noneable(field: ModelField) -> bool:
     if not field.required:
         # Taken from [Pydantic](https://github.com/samuelcolvin/pydantic/blob/v1.8.2/pydantic/fields.py#L946-L947)
-        is_optional = field.allow_none and (
+        return field.allow_none and (
             field.shape != SHAPE_SINGLETON or not field.sub_fields
         )
-        return is_optional and field.default is None and field.default_factory is None
     return False
diff --git a/tests/test_nullable.py b/tests/test_nullable.py
new file mode 100644
index 0000000..1c8b37b
--- /dev/null
+++ b/tests/test_nullable.py
@@ -0,0 +1,125 @@
+from typing import Optional
+
+import pytest
+from sqlalchemy.exc import IntegrityError
+from sqlmodel import Field, Session, SQLModel, create_engine
+
+
+def test_nullable_fields(clear_sqlmodel, caplog):
+    class Hero(SQLModel, table=True):
+        primary_key: Optional[int] = Field(
+            default=None,
+            primary_key=True,
+        )
+        required_value: str
+        optional_default_ellipsis: Optional[str] = Field(default=...)
+        optional_default_none: Optional[str] = Field(default=None)
+        optional_non_nullable: Optional[str] = Field(
+            nullable=False,
+        )
+        optional_nullable: Optional[str] = Field(
+            nullable=True,
+        )
+        optional_default_ellipses_non_nullable: Optional[str] = Field(
+            default=...,
+            nullable=False,
+        )
+        optional_default_ellipses_nullable: Optional[str] = Field(
+            default=...,
+            nullable=True,
+        )
+        optional_default_none_non_nullable: Optional[str] = Field(
+            default=None,
+            nullable=False,
+        )
+        optional_default_none_nullable: Optional[str] = Field(
+            default=None,
+            nullable=True,
+        )
+        default_ellipses_non_nullable: str = Field(default=..., nullable=False)
+        optional_default_str: Optional[str] = "default"
+        optional_default_str_non_nullable: Optional[str] = Field(
+            default="default", nullable=False
+        )
+        optional_default_str_nullable: Optional[str] = Field(
+            default="default", nullable=True
+        )
+        str_default_str: str = "default"
+        str_default_str_non_nullable: str = Field(default="default", nullable=False)
+        str_default_str_nullable: str = Field(default="default", nullable=True)
+        str_default_ellipsis_non_nullable: str = Field(default=..., nullable=False)
+        str_default_ellipsis_nullable: str = Field(default=..., nullable=True)
+
+    engine = create_engine("sqlite://", echo=True)
+    SQLModel.metadata.create_all(engine)
+
+    create_table_log = [
+        message for message in caplog.messages if "CREATE TABLE hero" in message
+    ][0]
+    assert "primary_key INTEGER NOT NULL," in create_table_log
+    assert "required_value VARCHAR NOT NULL," in create_table_log
+    assert "optional_default_ellipsis VARCHAR NOT NULL," in create_table_log
+    assert "optional_default_none VARCHAR," in create_table_log
+    assert "optional_non_nullable VARCHAR NOT NULL," in create_table_log
+    assert "optional_nullable VARCHAR," in create_table_log
+    assert (
+        "optional_default_ellipses_non_nullable VARCHAR NOT NULL," in create_table_log
+    )
+    assert "optional_default_ellipses_nullable VARCHAR," in create_table_log
+    assert "optional_default_none_non_nullable VARCHAR NOT NULL," in create_table_log
+    assert "optional_default_none_nullable VARCHAR," in create_table_log
+    assert "default_ellipses_non_nullable VARCHAR NOT NULL," in create_table_log
+    assert "optional_default_str VARCHAR," in create_table_log
+    assert "optional_default_str_non_nullable VARCHAR NOT NULL," in create_table_log
+    assert "optional_default_str_nullable VARCHAR," in create_table_log
+    assert "str_default_str VARCHAR NOT NULL," in create_table_log
+    assert "str_default_str_non_nullable VARCHAR NOT NULL," in create_table_log
+    assert "str_default_str_nullable VARCHAR," in create_table_log
+    assert "str_default_ellipsis_non_nullable VARCHAR NOT NULL," in create_table_log
+    assert "str_default_ellipsis_nullable VARCHAR," in create_table_log
+
+
+# Test for regression in https://github.com/tiangolo/sqlmodel/issues/420
+def test_non_nullable_optional_field_with_no_default_set(clear_sqlmodel, caplog):
+    class Hero(SQLModel, table=True):
+        primary_key: Optional[int] = Field(
+            default=None,
+            primary_key=True,
+        )
+
+        optional_non_nullable_no_default: Optional[str] = Field(nullable=False)
+
+    engine = create_engine("sqlite://", echo=True)
+    SQLModel.metadata.create_all(engine)
+
+    create_table_log = [
+        message for message in caplog.messages if "CREATE TABLE hero" in message
+    ][0]
+    assert "primary_key INTEGER NOT NULL," in create_table_log
+    assert "optional_non_nullable_no_default VARCHAR NOT NULL," in create_table_log
+
+    # We can create a hero with `None` set for the optional non-nullable field
+    hero = Hero(primary_key=123, optional_non_nullable_no_default=None)
+    # But we cannot commit it.
+    with Session(engine) as session:
+        session.add(hero)
+        with pytest.raises(IntegrityError):
+            session.commit()
+
+
+def test_nullable_primary_key(clear_sqlmodel, caplog):
+    # Probably the weirdest corner case, it shouldn't happen anywhere, but let's test it
+    class Hero(SQLModel, table=True):
+        nullable_integer_primary_key: Optional[int] = Field(
+            default=None,
+            primary_key=True,
+            nullable=True,
+        )
+
+    engine = create_engine("sqlite://", echo=True)
+    SQLModel.metadata.create_all(engine)
+
+    create_table_log = [
+        message for message in caplog.messages if "CREATE TABLE hero" in message
+    ][0]
+    assert "nullable_integer_primary_key INTEGER," in create_table_log