Abstract: A viewable thermostat device for providing visual confirmation of the proper operation of a thermostat in operating engines having a fluid coolant is provided by the utilization of a transparent thermoplastic housing having means for capturing the annular flange of the thermostat and maintaining the thermostat perpendicular to the direction of flow of the coolant while allowing a visual confirmation of the proper operation of the thermostat. The transparent thermoplastic housing is preferably formed from a transparent polyurethane or polycarbonate plastic and may include curved surfaces for optically enhancing the visual inspection of the operation of the thermostat as well as covers or sleeves for protecting the clear plastic from exterior abrasion or dirt. The viewable thermostat device is applicable to in-line installation as well as installations directly connected to the engine block to provide visual confirmation of the proper operation of the thermostat during engine operation.