Black phosphorus (BP) is a promising 2D material that has attracted tremendous attention owing to its high carrier mobility and large tunable bandgap from the visible to mid-infrared range. However, due to the unstable lattice structure, monolayer BP is highly sensitive to oxygen and water, and its delicate monolayer channel could be totally destroyed during conventional fabrication process (involving multiple solution steps). Therefore, even after a decade of intense input, the monolayer BP transistor has yet to be realized, and its fundamental electrical and optoelectronic properties remain largely unknown. Here, we report a van der Waals integration method to realize pure monolayer BP devices and investigate its electrical and optical properties. We found the carrier mobility of BP drops sharply with channel thickness down to monolayer, which is more like a conventional bulk semiconductor rather than a layered vdW semiconductor. Furthermore, the optoelectronic performance of the monolayer black phosphorus transistor has also been investigated, exhibiting a large photoresponsivity of 17 A/W and a detectivity of 3.1 × 10⁸ Jones. Our work first realized pure monolayer BP devices and investigated their intrinsic electrical and optoelectronic properties, which could open new possibilities for the exploration of rich physics within monolayer BP as well as other unstable semiconductors.