A power supply adapter with a foldable plug is disclosed. The power supply adapter includes a first casing, a second casing, a rotatable cylinder, a projecting plug, and a lighter connector. A cavity is formed on the front end of the first casing. Two vertical plates are mounted on the second casing for supporting the rotatable cylinder. A lighter connector is connected to the front end of the projecting plug for insertion into a lighter socket. The projecting plug is formed on the front end of the rotatable cylinder and extends from a cavity space defined by the front end of the first casing and the second casing. By rotating the rotatable cylinder, the angle of the projecting plug is changed. A voltage converting circuit is arranged in the adapter for supplying a first and a second output voltage.