In this paper, a simple multimode hybrid energy storage system (HESS) is proposed for electric vehicles (EVs). Compared to the improved semiactive HESS, only two switches are added in the main circuit topology of the multimode HESS, thereby achieving the operating modes can be actively switched. The mode switch strategy is designed according to the driving modes of the EV and the status of the power sources. To improve the overall system efficiency of the multimode HESS, the boost converter will operate at the peak efficiency to convey the energy from the battery to the supercapacitor (SC). An adaptive sliding-mode control (ASMC) with hysteresis control (HC) strategy is also developed by combining practical application of the multimode HESS. Simulations and experiments are presented to verify the effectiveness of the proposed multimode HESS and its ASMC strategy. Simulated results show that the multimode HESS can select suitable operating modes in corresponding conditions. Compared to the total sliding-mode control strategy, experimental results demonstrate that the ASMC with HC strategy can improve the operating stability of the multimode HESS under different operating modes. The multimode HESS can not only switch to suitable operating modes, but also avoid the excessive output power of the battery to meet different power demands of the load. In addition, the SC absorbs all the braking energy such that the battery safety can be effectively ensured.