An automatic gas shutoff system for protecting the occupants of a home, apartment, RV, vehicle, etc. from dangerous levels of carbon monoxide. The automatic gas shutoff system generally includes one or more carbon monoxide detectors, a control unit, and a gas shutoff valve under the control of the control unit, so that the gas supply to a malfunctioning appliance can be shut off in the event it is creating dangerous levels of carbon monoxide within a protected space. The shutoff valve may have a manual reset feature that requires a problem condition to be repaired before the valve is manually reset to allow normal gas flow to resume.