An integrated resonant tunneling diode (RTD)-based 4: 1 multiplexer core for low power consumption and high-speed operation is proposed. The proposed 4: 1 multiplexer core is designed based on a power-efficient negative differential resistance (NDR) circuit topology, which actively utilizes the unique NDR characteristics of the RTD. The proposed IC is comprised of two RTD-based 2: 1 multiplexers and a 2: 1 selector. The designed IC has been fabricated using an InP RTD/heterojunction bipolar transistor (HBT) monolithic microwave integrated circuit technology, which is optimized by introducing an undercut process in the stacked RTD/HBT epistructure. A low power consumption of 75 mW at a supply voltage of -2.9 V has been achieved at a speed up to 40 Gb/s. The implemented IC, which has a higher complexity than monolithically integrated RTD/transistor digital circuits reported to date, is the first demonstration of a low-power high-speed 4: 1 multiplexer IC based on an NDR device technology.