A cooking appliance having a ventilation system is provided herein. The cooking appliance may include a cabinet, a primary air passage, a fan, and a ventilation sensor. The cabinet may include a top panel and a bottom panel. The cabinet may extend along a vertical direction between the top panel and the bottom panel. The primary air passage may be defined between the top panel and the bottom panel. The primary air passage may extend along a transverse direction between a primary inlet and an exhaust port. The fan may be positioned in fluid communication with the primary air passage downstream from the primary inlet and upstream from the exhaust port. The ventilation sensor may be positioned within the cabinet upstream from the fan and in fluid communication therewith.