Metamaterials have been designed to achieve a wide range of functionalities. Metamaterial absorbers are of particular interest for various applications such as infrared detectors, emissivity coatings, and photovoltaic cells. Various metamaterial platforms have been demonstrated to achieve perfect absorption and several attempts have been made to extend the absorption bandwidth of such devices. We demonstrate a broadband infrared absorber using an asymmetric Fabry–Perot cavity consisting of a monolithically fabricated two-layer metasurface. Superoctave optical absorption is achieved by tailoring the structure of the metasurface layers and the thickness of the cavity. The device yields absorptance of over 80% from λ = 4–16 μm, while maintaining the performance over a wide range of incident angles. In contrast to most metamaterial absorbers, our metasurface layers are made of customized indium tin oxide (ITO), conferring the advantage of CMOS compatibility compared to previous approaches using noble metals.