Plasmonic devices enable nanophotonics and nanodevices as an alternative to address the miniaturization and diffraction limit problems in photonics devices. An all-optical 2  ×  1 multiplexer (MUX) based on aplasmonic multilayer structure (insulator–metal–insulator) is proposed. The working wavelength is 1550 nm and the design area is (350  nm  ×  250  nm). The proposed device works under the principle of destructive and constructive interference, which happens between inputs and selector signals. The performance of the proposed device is measured by three criteria: transmission, extension ratio, and modulation depth. In transmission signals, an amplifying process occurs; two states are exceeding 100%. The proposed structure dimensions are excellent and optimum according to the value of MD (98.85%). In addition, the operation of the proposed structure is achieved without the need for the control signal, which adds more complicity and degrades the device efficiency. The proposed structure has a major role in building the all-optical arithmetic logic unit, photonics integrated circuits, and THz applications.