We present a 1 × 1 multimode optical switch for E_11, E_21, E_12, and E_22 modes based on cascaded Mach–Zehnder interferometer (MZI) waveguides, where the primary MZI is used to split E_11, E_21, E_12, and E_22 modes into E_11 or E_12 mode and then couple back to the original mode at the output, and the secondary MZIs are the modulation arms of the primary MZI. In addition, the secondary MZIs are designed to be mode-insensitive for switching E_11 and E_12 modes simultaneously. As a proof of concept, we fabricate the device with polymer material to achieve thermo-optic switching for the four modes. Our experimental device exhibits the extinction ratios of larger than 10.2 dB with a power consumption of 5.5 mW and response times of less than 1.28 ms for each mode. The presented device can be widely applied in mode-division multiplexing (MDM) systems where multimode switching is needed.