Molybdenum disulfide (MoS 2 )-based phototransistors are attractive for optical electronics in a large-scale size, such as transparent touch screens. However, most of the work done over the past decade has been on an opaque SiO 2 ∕Si wafer with a small size (micrometer to millimeter). In this work, a large-scale multilayer MoS 2 -based phototransistor has been fabricated on a transparent freestanding gallium nitride (GaN) wafer using a scalable chemical vapor deposition method. Due to the near lattice match and small thermal expansion mismatch between GaN and MoS 2 , the as-grown multilayer MoS 2 -on-GaN film shows high material quality in terms of low full width at half-maximum (∼5.16 cm −1 ) for the E 1 2g Raman mode and a high absorption coefficient (∼10 6 cm −1 ) in the wavelength range of 405–638 nm. Under a wavelength of 405 nm at an incident power of 2 mW and applied voltage of 9 V, the fabricated MoS 2 -on-GaN phototransistor achieved a maximum responsivity of 17.2 A/W, a photocurrent gain of 53.6, and an external quantum efficiency of 5289%, with specific detectivity (∼10 10 –10 12 Jones) and low noise equivalent power (10 −12 –10 −14 W∕Hz 1∕2 ) in the visible range of 405–638 nm. A typical response time of 0.1–4 s in the ambient air has also been recorded for the demonstrated MoS 2 -on-GaN phototransistor. Our work paves a technologic stepping stone for MoS 2 -based phototransistors for multifunctional transparent and touch-based optoelectronics in the future.