Active materials play a crucial role in the performance of phototransistors. However, the discovery of a novel and versatile active material is a big challenge. For the first time, phototransistors with ultrathin niobium-doped indium oxide (InNbO) active layer are fabricated. The InNbO phototransistors without additional light-absorbing layers exhibit the performance with a high average mobility of 22.86 cm Vs, a turn-on voltage of −0.75 V, a low sub threshold swing of 0.18 V/decade, and a high on/off current ratio of 5.74 × 10. Detailed studies show that Nb is the key to suppress the free carrier generation due to the strong bonding strength of Nb─O. In addition, the InNbO phototransistors exhibit a very broad spectral responsivity with a photocurrent of 4.72 × 10 A, a photosensitivity of 1.69 × 10, and a high detectivity of 3.33 × 10 Jones under violet (405 nm) light illumination, which is significantly higher than that of the IGZO phototransistors. Furthermore, an active-matrix quantum-dot light-emitting diode pixel circuit based on InNbO phototransistors is demonstrated. The findings not only indicate that InNbO is a new active material for phototransistors, but also suggest that InNbO-based phototransistors have a great potential for the next-generation interactive display technology.