We present an ultra-thin lateral SOI PIN photodiode with transferred monolayer graphene as the transparent gate, to provide enhanced ultraviolet (UV) performance and mechanical flexibility beyond standard Si-based devices. The device dark current shows intact characteristics after the post-CMOS thinning and graphene transfer processing steps. The device responsivity presents high potential in UV and visible wavelength detections (i.e. within the 200-900 nm range) under monochromatic light illumination. A maximum responsivity of 0.18 A W-1 has been experimentally achieved at 390 nm wavelength and validated by simulation, for a diode with intrinsic length L i of 20 μm. Additionally, the ∼5 μm-thick device chip with direct board assembly paves the way towards the development of hybrid flexible electronics.