A fan module and a hat with a fan module are disclosed. The fan module includes a case, a fan, and an air guide tube. The case includes a connecting portion, wherein the case is detachably engaged with a visor of a hat. The case has a receiving space and an air outlet, wherein the air outlet communicates with the receiving space. The fan is rotatably provided in the receiving space of the case. An airflow is generated when the fan rotates, wherein the airflow is exhausted out of the case through the air outlet. The air guide tube is connected to the case, and guides the airflow to a space between the head of a wearer and the main body. Whereby, the hat with the fan module could provide not only sunscreen protection, but also a cooling effect.