diff --git a/growthbook/common_types.py b/growthbook/common_types.py
index 0e167e9..6a01a6a 100644
--- a/growthbook/common_types.py
+++ b/growthbook/common_types.py
@@ -54,6 +54,7 @@ def __init__(
         bucketVersion: Optional[int] = None,
         minBucketVersion: Optional[int] = None,
         parentConditions: Optional[List[Dict[str, Any]]] = None,
+        customFields: Optional[Dict[str, Any]] = None,
         **_ignored: Any,
     ) -> None:
         self.key = key
@@ -76,6 +77,9 @@ def __init__(
         self.bucketVersion = bucketVersion or 0
         self.minBucketVersion = minBucketVersion or 0
         self.parentConditions = parentConditions
+        # Custom Fields defined for the experiment in the GrowthBook UI.
+        # Arrives from the API as a flat dict (e.g. {"cfl_abc123": "value"}).
+        self.customFields = customFields or {}
 
         self.fallbackAttribute = None
         if not self.disableStickyBucketing:
@@ -117,6 +121,8 @@ def to_dict(self) -> Dict[str, Any]:
             obj["minBucketVersion"] = self.minBucketVersion
         if self.parentConditions:
             obj["parentConditions"] = self.parentConditions
+        if self.customFields:
+            obj["customFields"] = self.customFields
 
         return obj
 
diff --git a/tests/test_growthbook.py b/tests/test_growthbook.py
index b51f6a6..a0b80c1 100644
--- a/tests/test_growthbook.py
+++ b/tests/test_growthbook.py
@@ -359,6 +359,38 @@ def test_handles_weird_experiment_values():
     gb.destroy()
 
 
+def test_custom_fields_parsed_from_api_dict():
+    # The API delivers experiment Custom Fields as a flat dict.
+    exp = Experiment(
+        **{
+            "key": "my-experiment",
+            "variations": ["control", "variant"],
+            "customFields": {"cfl_abc123": "My custom field", "cfl_def456": 42},
+        }
+    )
+    assert exp.customFields == {"cfl_abc123": "My custom field", "cfl_def456": 42}
+    assert exp.to_dict()["customFields"] == {
+        "cfl_abc123": "My custom field",
+        "cfl_def456": 42,
+    }
+
+
+def test_custom_fields_default_when_absent():
+    exp = Experiment(key="my-experiment", variations=["control", "variant"])
+    assert exp.customFields == {}
+    # Empty custom fields are omitted from the serialized dict.
+    assert "customFields" not in exp.to_dict()
+
+
+def test_custom_fields_public_init():
+    exp = Experiment(
+        key="my-experiment",
+        variations=["control", "variant"],
+        customFields={"cfl_xyz": "hello"},
+    )
+    assert exp.customFields == {"cfl_xyz": "hello"}
+
+
 def test_skip_all_experiments_flag():
     """Test that skip_all_experiments flag prevents users from being put into experiments"""