2D graphitic carbon nitride (g-C₃N₄) nanosheets (CNNS) with a mean thickness of 2 nm were successfully synthesized and compounded into poly-ether-ether-ketone (PEEK). Adding low-loading CNNS into PEEK decreased the crystallinity of the matrix, whereas the hardness was enhanced. Moreover, the addition of CNNS increased the wettability of synthetic oil in contact with PEEK materials, e.g. the contact angles decreased from 13.8° to 5.5° when 2.0 vol% CNNS was added into PEEK. Our work demonstrated that the nanosheets improved dramatically the wear resistance of PEEK subjected to boundary and mixed lubrication conditions. In particular, wear reduction of PEEK achieved by addition of 0.5 vol% CNNS was comparable to that obtained by filling with 20-fold volume fractions of carbon fibers or microsized g-C₃N₄ particles. Comprehensive interface analyses indicated that a robust tribofilm containing significant fraction of CNNS nanosheets was generated, enhancing boundary lubricity by mitigating direct rubbing of the tribo-pair.