•Black phosphorus dotted with Ag nanoparticles is synthesized by a facial and in-suit chemical reduction approach.•The nanocomposite of Ag/BP is an excellent lubricant additive achieving ultralow friction and excellent wear resistance.•Ag/BP can act as a catalyst to decompose the base PAO oil forming a carbon-based tribofilm during rubbing process.•The formed tribofilm consisting of BP and amorphous carbon corresponds to the superior lubricating ability of Ag/BP. Black phosphorus (BP) is a new two-dimensional (2D) material with attractive nanoscale chemical and physical properties. Herein, black phosphorus dotted with silver nanoparticles (Ag/BP) was synthesized via a facile and in-suit chemical reduction approach without reductant. The tribological properties of as-prepared Ag/BP nanocomposite as lubricant additive in poly-alpha-olefin (PAO6) based oil were investigated by a ball-on-disc tribometer. The results demonstrate that the oil dispersed with trace amount of Ag/BP nanoadditives shows excellent lubricating performance for the steel/steel contact. The friction coefficient and wear rate of the oil dispersed with 0.075 wt% Ag/BP nanoadditives are reduced by 73.4% and 92.0% respectively, as compared to the based PAO6 oil. The possible friction-reducing and anti-wear mechanisms of Ag/BP nanoadditives in oil are illustrated by analyzing the wear scar surfaces. During scratching, the Ag/BP nanomaterials serves not only as a friction-reducing and anti-wear additive, but also as a catalyst to decompose the based PAO oil for forming a carbon-based tribofilm under ambient condition that further reduces the friction and wear. The findings here provide a novel insight into the design of lubricant additive incorporated in base oil and suggests a new potential application of black phosphorus in lubrication and tribology.