This paper presents a fault-tolerant powertrain topology for series hybrid electric vehicles (SHEVs). The introduction of a redundant phase leg that is shared by three converters in a standard SHEV drive system allows to maximize the reliability improvement with minimal part-count increase. The new topology features fast response in fault detection and isolation, and post-fault operation at rated power throughput. The operating principle, control strategy, and fault diagnostic methods are elaborated. The substantially improved reliability over the standard topology is verified by the Markov reliability model. Time-domain simulation based on a Saber model has been conducted and the results have verified the feasibility and performance of the proposed SHEV drive system with fault-tolerant capability. The experimental results from a prototype have further validated the robust fault detection scheme and excellent post-fault performance.