The Martian dynamo evolution is critical for understanding Mars’s interior structure, thermal evolution, and climate change. It has been inferred to shut down at ∼4.1–4.0 Ga based on the magnetic signatures of large impact craters, but be present at ∼3.9 Ga and ∼3.7 Ga from the paleomagnetic studies and magnetic fields above volcanic units. Here, we investigate the magnetic signatures of the Iota crater, located inside the CT3-G area with a centrally strong magnetic anomaly. The Iota crater shows a weak central magnetic field with an inside-outside strength ratio of 0.39. Forward modeling is established to explore the relationship between the magnetic field signatures of craters and the magnetization caused by impact. The results show that the average magnetization of the retained materials beneath the Iota crater is about 20% of the maximum of the surroundings, indicating that the dynamo strength at that time became weak. The magnetic signatures of Iota and CT3-G reveal that the Martian dynamo decayed at ∼4.1 Ga, but did not stop completely.