The photoelectronic characteristics of single-crystalline nanowire organic phototransistors (NW-OPTs) are studied using a high-performance n-channel organic semiconductor, N,N′-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI), as the photoactive layer. The optoelectronic performances of the NW-OPTs are analyzed by way of their current–voltage (I–V) characteristics on irradiation at different wavelengths, and comparison with corresponding thin-film organic phototransistors (OPTs). Significant enhancement in the charge-carrier mobility of NW-OPTs is observed upon light irradiation as compared with when performed in the dark. A mobility enhancement is observed when the incident optical power density increases and the wavelength of the light source matches the light-absorption range of the photoactive material. The photoswitching ratio is strongly dependent upon the incident optical power density, whereas the photoresponsivity is more dependent on matching the light-source wavelength with the maximum absorption range of the photoactive material. BPE-PTCDI NW-OPTs exhibit much higher external quantum efficiency (EQE) values (≈7900 times larger) than thin-film OPTs, with a maximum EQE of 263 000%. This is attributed to the intrinsically defect-free single-crystalline nature of the BPE-PTCDI NWs. In addition, an approach is devised to analyze the charge-transport behaviors using charge accumulation/release rates from deep traps under on/off switching of external light sources.