A monolithic optical coherent receiver with quadrature detection is presented for the first time in an unmodified bulk CMOS chip. Networks of optical phase shifters and splitters distribute modulated light across the chip using subtractive photonic waveguides operating at 780 nm, and integrated avalanche photodiodes and transimpedance amplifiers enable balanced detection and amplification in the electronic domain. All components are monolithically integrated in an unmodified 180 nm bulk CMOS process with simple post-processing steps, paving the way for low-cost optoelectronic imaging and sensing systems utilizing coherent detection.