diff --git a/doc/changelog.rst b/doc/changelog.rst
index 7c2dae2..93d7a3c 100644
--- a/doc/changelog.rst
+++ b/doc/changelog.rst
@@ -1,6 +1,13 @@
 Changelog
 =========
 
+[0.4.3] - Unreleased
+--------------------
+
+Fixed
+^^^^^
+- Only allow one primary complex attribute value to be true. :issue:`10`
+
 [0.4.2] - 2025-08-05
 --------------------
 
diff --git a/scim2_models/base.py b/scim2_models/base.py
index 53c5d98..218cfb9 100644
--- a/scim2_models/base.py
+++ b/scim2_models/base.py
@@ -355,6 +355,55 @@ def check_replacement_request_mutability(
             cls._check_mutability_issues(original, obj)
         return obj
 
+    @model_validator(mode="after")
+    def check_primary_attribute_uniqueness(self, info: ValidationInfo) -> Self:
+        """Validate that only one attribute can be marked as primary in multi-valued lists.
+
+        Per RFC 7643 Section 2.4: The primary attribute value 'true' MUST appear no more than once.
+        """
+        from scim2_models.attributes import MultiValuedComplexAttribute
+
+        scim_context = info.context.get("scim") if info.context else None
+        if not scim_context:
+            return self
+
+        for field_name in self.__class__.model_fields:
+            # Check if field is multi-valued (list type)
+            if not self.get_field_multiplicity(field_name):
+                continue
+
+            field_value = getattr(self, field_name)
+            if field_value is None:
+                continue
+
+            # Check if items in the list have a 'primary' attribute
+            element_type = self.get_field_root_type(field_name)
+            if (
+                element_type is None
+                or not isclass(element_type)
+                or not issubclass(element_type, MultiValuedComplexAttribute)
+            ):
+                continue
+
+            primary_count = sum(
+                1
+                for item in field_value
+                if isinstance(item, PydanticBaseModel)
+                and getattr(item, "primary", None) is True
+            )
+
+            if primary_count > 1:
+                raise PydanticCustomError(
+                    "primary_uniqueness_error",
+                    "Field '{field_name}' has {count} items marked as primary, but only one is allowed per RFC 7643",
+                    {
+                        "field_name": field_name,
+                        "count": primary_count,
+                    },
+                )
+
+        return self
+
     @classmethod
     def _check_mutability_issues(
         cls, original: "BaseModel", replacement: "BaseModel"
diff --git a/tests/test_user.py b/tests/test_user.py
index 656f26d..aa1b602 100644
--- a/tests/test_user.py
+++ b/tests/test_user.py
@@ -124,3 +124,65 @@ def test_full_user(load_sample):
         obj.meta.location
         == "https://example.com/v2/Users/2819c223-7f76-453a-919d-413861904646"
     )
+
+
+def test_primary_attribute_validation_valid_cases():
+    """Test primary attribute validation for valid cases (0 or 1 primary)."""
+    from scim2_models.context import Context
+
+    # Case 1: No primary attributes
+    user_data = {
+        "schemas": ["urn:ietf:params:scim:schemas:core:2.0:User"],
+        "userName": "testuser",
+        "emails": [
+            {"value": "test1@example.com", "type": "work"},
+            {"value": "test2@example.com", "type": "home"},
+        ],
+    }
+    user = User.model_validate(
+        user_data, context={"scim": Context.RESOURCE_CREATION_REQUEST}
+    )
+    assert user.user_name == "testuser"
+
+    # Case 2: Exactly one primary attribute
+    user_data_with_primary = {
+        "schemas": ["urn:ietf:params:scim:schemas:core:2.0:User"],
+        "userName": "testuser2",
+        "emails": [
+            {"value": "primary@example.com", "type": "work", "primary": True},
+            {"value": "secondary@example.com", "type": "home", "primary": False},
+        ],
+    }
+    user_with_primary = User.model_validate(
+        user_data_with_primary, context={"scim": Context.RESOURCE_CREATION_REQUEST}
+    )
+    assert user_with_primary.emails[0].primary is True
+    assert user_with_primary.emails[1].primary is False
+
+
+def test_primary_attribute_validation_invalid_case():
+    """Test primary attribute validation for invalid case (multiple primary)."""
+    import pytest
+    from pydantic import ValidationError
+
+    from scim2_models.context import Context
+
+    # Case: Multiple primary attributes (should fail)
+    user_data_invalid = {
+        "schemas": ["urn:ietf:params:scim:schemas:core:2.0:User"],
+        "userName": "testuser3",
+        "emails": [
+            {"value": "primary1@example.com", "type": "work", "primary": True},
+            {"value": "primary2@example.com", "type": "home", "primary": True},
+        ],
+    }
+
+    with pytest.raises(ValidationError) as exc_info:
+        User.model_validate(
+            user_data_invalid, context={"scim": Context.RESOURCE_CREATION_REQUEST}
+        )
+
+    error = exc_info.value.errors()[0]
+    assert error["type"] == "primary_uniqueness_error"
+    assert "emails" in error["ctx"]["field_name"]
+    assert error["ctx"]["count"] == 2