Room-temperature mid- and far-infrared photodetectors and energy harvesters meet diverse upcoming demands including health condition monitoring, industrial inspection, and miniaturized power-source for Internet of Things. However, the cryogenic cooling requirement for III-V semiconductors and the inefficient light absorption in two-dimensional (2D) materials, for example, graphene (2.3%) and black phosphorus (∼3%), have hindered mid- and far-infrared optoelectronics from widespread applications. Here, we demonstrate a self-powered infrared photodetector as well as energy harvester employing vertical photothermoelectric (PTE) effect of a carbon nanotube forest (CNTF). In the self-assembled anti-reflecting CNTF, 99.4% reflection suppression is observed, resulting in a broadband detectivity of 1.9 × 10 cm Hz in 2.5-25 μm spectral range and peak detectivity of 2.3 × 10 cm Hz at 4.3 THz nonlithography fabrication. By virtue of vertical architecture, this photodetector exhibits enhanced sensitivity to weak and unfocused infrared illumination, which mitigates the high actuating power density in conventional PTE or field-effect detectors and renders practical infrared detection in the real life.