Broadband response is pursued in both infrared (IR) and terahertz (THz) detection technologies, which find their applications in both terrestrial and astronomical realms. Herein, we report an ultrabroadband and multiband IR/THz detector based on blocked-impurity-band detecting principle. The detectors are prepared by implanting phosphorus into germanium (Ge:P), where photoresponses with a P impurity band, a self-interstitial defect band, and a vacancy-P (V-P) pair defect band are realized simultaneously. The response spectra of the detectors show ultrabroad and dual response bands in a range of 3–28 μm (IR band) and 40–165 μm (THz band), respectively. Additionally, a tiny mid-IR (MIR) band within 3–4.2 μm is embedded in the IR band. The THz band arises from the P impurity band, whereas the IR and the MIR bands are ascribed to the two defect bands. At 150 mV and 4.5 K, the peak detectivities of the three bands are obtained as 2.9×1012 Jones (at 3.9 μm), 6.8×1012 Jones (at 16.3 μm), and 9.9×1012 Jones (at 116.5 μm), respectively. The impressive coverage and sensitivity of the detectors are promising for applications in IR and THz detection technologies.