To realize the sensitive and rapid sensing of humidity, 2D as-prepared TiO2 nanosheets (hereafter shorted as TiO2 nanosheets) with characteristics of surface oxygen vacancy defects and large surface area were designed and synthesized. The TiO2 nanosheets-based sensor exhibit superior humidity sensing performance with an ultrahigh sensitivity evidenced by the dramatic impedance variation of more than four orders of magnitude from relative humidity (RH) 11% to 95%, fast response (3 s) and recovery (50 s) process, as well as a small hysteresis (similar to 4.6%). Additionally, a systematic study of the sensing performances of sensors fabricated from TiO2 nanosheets and two counterparts calcined in N-2 and O-2 with decreased specific surface area and controlled surface defects, namely TiO2-400 (N-2) and TiO2-400 (O-2), has been conducted. The results show that at low RH level of 33% and 54%, TiO2-400 (N-2) with highest surface oxygen vacancy defects concentration exhibits the highest response of 6.26 and 34, respectively, while at high RH range of 85% to 95%, the TiO2 nanosheets with largest specific surface area shows the highest response which is almost 10 times higher than that of the calcined counterparts. Combining the complex impedance analysis, the overall ultrahigh humidity sensing performance of TiO2 nanosheets-based sensor is attributed to the dissociating promotion by the abundant surface defect and enhanced electrolytic conduction by the large specific surface area.