To improve the reliability of high speed permanent magnet machines (HSPMMs) under multiphysics constraints, including the electromagnetic properties, losses, rotor stress, rotor dynamics, and temperature, the rotor of an HSPMM is optimized to achieve low loss and temperature in this paper. To assess the impact of each rotor design parameter on multiphysics performance, a comprehensive sensitivity analysis of the rotor parameters on multiphysics performance is first implemented. On this basis, a multiphysics optimization process for HSPMM rotor is proposed to obtain the optimal design parameters. A comparison of the multiphysics performances of the initial and optimized design schemes shows that the optimized scheme can achieve much lower rotor loss and temperature. The optimization scheme is verified by comprehensive experimental tests on a 400 kW, 10 000 rpm HSPMM prototype.