Compared with the conventional three-phase motor, the five-phase interior permanent magnet synchronous motor (IPMSM) has better advantages on fault-tolerance due to redundant freedom. In order to obtain further fault-tolerance, this paper proposes a new high frequency (HF) signal-injection-based sensorless control strategy for five-phase IPMSM drives. The key is to extend the position sensorless technology based on HF signal injection into the five-phase motor control system, especially proposing a sensorless control method by injecting a HF square-wave voltage signal into the third harmonic space. The induced HF current responses in the third harmonic space can be utilized to track the rotor speed and position accurately. Compared with the conventional HF sinusoidal signal injection methods, the proposed method can effectively reduce the influence of digital filters, such as time delay and decreased performance because the low-pass-filter (LPF) is removed. In addition, the torque ripple caused by the proposed injection method in third harmonic space is reduced compared with the method in fundamental space. The effectiveness of the proposed method is verified by simulation and experimental results. INDEX TERMS Sensorless control, five-phase interior permanent magnet synchronous motor, third harmonic space, HF square-wave voltage injection (HF-SVI).