The chemical bulk reductive covalent functionalization of thin‐layer black phosphorus (BP) using BP intercalation compounds has been developed. Through effective reductive activation, covalent functionalization of the charged BP by reaction with organic alkyl halides is achieved. Functionalization was extensively demonstrated by means of several spectroscopic techniques and DFT calculations; the products showed higher functionalization degrees than those obtained by neutral routes. The reductive covalent functionalization of thin‐layer black phosphorus (BP) proceeds via BP intercalation compounds. Following the reductive activation of BP, the resulting charged BP reacts with alkyl halides and undergoes covalent functionalization. The BP lattice shows unprecedented opening and a higher degree of functionalization than that obtained by neutral routes.