Discovering ways to control magnetic states by light is very attractive for prospective applications of optical–magnetic sensing and recording. Although several studies have demonstrated the light-induced switching of magnetization, the magnetoresistance effect was seldom thought to depend on light illumination. A light-induced magnetoresistance sign switching the resulting writable optical memory are described. A practical method is developed to form a p-n junction at the interface of a p-type amorphous carbon film and a quasi-2D electron gas on an SrTiO3 surface. When illuminated, the as-formed junction exhibits a clear transition from positive to negative magnetoresistance. A change of optical intensity influences the response time, but not the final value of the negative magnetoresistance. It is also found that, after a light pulse, the negative magnetoresistance state tends to persist, indicating a longstanding memory for an optical signal. This points to a new direction for studying light-induced magnetoresistance switching, which could lead to the development of new memory devices.