An active-clamped three-level rectifier (AC-TLR) is derived from the diode-clamped three-level inverter, by replacing the active switches and the diodes in the three-level inverter with the diodes and active switches, respectively. Novel isolated buck-boost converters, featuring single-stage conversion and soft switching within wide operation range, are developed based on the proposed AC-TLR. By utilizing the AC-TLR, the voltage stress on the power devices and passive components, including the rectifying diodes, the active-clamping switches, the flying capacitor, and output filter capacitors, is reduced to the half of the output voltage. Low-voltage rating switching devices with better switching and conduction performances and a transformer with reduced turns ratio and parasitic parameters are used to enhance the efficiency. The full-bridge isolated buck-boost converter with the proposed AC-TLR is analyzed in detail as an example. An optimized phase shift control strategy is employed to realize isolated buck and boost conversion. Soft switching of all of the switching devices in both the primary- and secondary-side circuits is achieved within the whole operation range by using the proposed AC-TLR and the phase shift control strategy. Experimental results on a prototype with 380-V output verify the effectiveness of the proposed AC-TLR and its derived isolated buck-boost converters.