This article proposes a novel semimodular three-phase ac/dc structure for modular solid-state transformers. The proposed system employs single-stage indirect matrix-based ac/dc converters. A single full-bridge rectifier is used for each phase of the medium voltage grid so that each ac/dc module does not need a voltage rectifier. As a result, the number of semiconductor devices in the proposed system is reduced significantly compared with the other existing SST-based systems. A decentralized control scheme without high-speed communication has been developed to regulate the input and output currents while keeping the voltages of each module balance. A design of a 360-kW system has been carried out to evaluate the efficiency of the proposed system, which is 97.7% at rated power. With delta connection in three-phase system, a third harmonic current can be injected to the phase currents to increase the output current and power by 15%. Simulation results for a 360-kW 3.3-kV system are provided to verify the performance of the proposed system. Finally, a 3-kW hardware prototype has been built and tested to demonstrate the feasibility of the proposed system.