The discovery of a giant piezoresistive effect in a semiconductor heterojunction by optoelectronic coupling can open a new era for mechanical sensors. This paper develops a novel concept of opto-electronic coupling in semiconductor heterojunctions for pressure sensing. We employ non-uniform illumination of visible light on a SiC/Si heterojunction to generate a gradient of charge carriers in the SiC nanofilm. These charge carriers are then manipulated by a tuning current, producing giant relative resistance changes in the material under applied pressure. We successfully demonstrated the enhancement by opto-electronic coupling in a SiC/Si heterojunction pressure sensor of sensitivity up to 185 000 times compared to the unilluminated condition. In addition, the opto-electronic coupling enables significantly improved repeatability, stability, signal-to-noise ratio and detectable range of the pressure sensor. The ultrahigh sensitive pressure sensing mechanism by opto-electronic coupling will pave a way for development of extremely sensitive mechanical sensors.