A jackknife warning system for a trailered vehicle. A rotation sensor, mounted to a chassis of a tractor, engages an underside of a semi-detached trailer and senses rotation of the trailer with respect to the tractor. A pressure sensor senses pressure applied to a set of brakes for the vehicle. When rotation of the trailer reaches a first setpoint, and pressure applied to the brakes reaches a second setpoint, an alarm is emitted. The alarm can either be a right alarm or a left alarm, depending on if the vehicle is jackknifing to the right or to the left. The brake pressure setpoint can be adjusted by the driver based on the load being carried or on road conditions.