By reassembling thin isolated atomic planes of hexagonal borum nitride (hBN) with a few layer phosphorene black phosphorus (BP), hBN/BP/hBN heterostructures are mechanically stacked to devise high‐efficiency THz photodetectors operating in the 0.3–0.65 THz range, from 4 K to 300 K, with a record signal‐to‐noise ratio of 20 000.