A thin-film perfect electromagnetic absorber with a tunable response in the infrared (IR) region is proposed using a metal-dielectric-metal configuration, which consists of a Ti top layer and a Ge spacer layer on a Ti substrate. The thin-film structure simplifies the absorber design by tuning the thicknesses of the two layers, which is suitable for large-scale fabrication by matured deposition technologies. The absorber supports perfect IR absorption with tunability from 3 µm to over 15 µm. Furthermore, the total thickness is much smaller than the wavelength, and the absorption has small iridescence. Based on this design, we demonstrated two samples with one supporting single-band absorption in the atmospheric absorption window (5-8 µm) and the other one supporting dual-band absorption in the two atmospheric transmission windows (3-5 and 8-13 µm). These absorption signatures can find applications in IR invisibility and radiative cooling.