Integration of two-dimensional (2D) materials with three-dimensional (3D) semiconductors leads to intriguing optical and electrical properties that surpass those of the original materials. Here, we report the high performance of a GaN nanopillar-modified graphene/GaN/Ti/Au photodetector (PD). After etching on the surface of a GaN film, GaN nanopillars exhibit multiple functions for improving the detection performance of the PD. Under dark conditions, surface etching reduces the contact area of GaN with the graphene electrode, leading to a reduced dark current for the PD. When illuminated with UV light, the nanopillars enable an enhanced and localized electric field inside GaN, resulting in an ∼20% UV light absorption enhancement and a several-fold increased photocurrent. In addition, the nanopillars are intentionally etched beneath the metal Ti/Au electrode to modify the semiconductor-metal junction. Further investigation shows that the modified GaN/Ti/Au contact triggers a prominent rectifying I-V behaviour. Benefiting from the nanopillar modification, the proposed PD shows a record large detectivity of 1.85 × 1017 Jones, a small dark current of 5.2 nA at +3 V bias, and a nearly three order of magnitude rectification ratio enhancement compared with non-nanopillar PDs. This pioneering work provides a novel nanostructure-modifying method for combining 2D materials and 3D semiconductors to improve the performances of electronic and optoelectronic devices.