Hydroxyl-functional hexagonal boron nitride nanosheets (OH-BNNSs) were prepared successfully by ultrasonic treatment, and then homogeneous dispersions of OH-BNNSs in 1,4-butanediol (BDO) were obtained. Various techniques were applied to characterize the OH-BNNSs and their hydroxyl functional groupd. Furthermore, biodegradable poly(butylene succinate) (PBS)/OH-BNNS nanocomposites were prepared by in situ polymerization of succinate acid (SA) and BDO containing well-dispersed OH-BNNS. The nucleating effect of low OH-BNNS loadings on PBS in the PBS/OH-BNNS nanocomposites was investigated. It is found that OH-BNNSs significantly improve the melt-crystallization temperature and degree of crystallinity of PBS during the nonisothermal crystallization process without changing the crystal structure of PBS. Moreover, the Avrami exponent n calculated for isothermal crystallization increased from 2 to 3 after the addition of 0.05 wt % OH-BNNSs. The nucleation density of PBS spherulites in the nanocomposites increased dramatically. These results demonstrate that OH-BNNSs function as an outstanding nucleating agent of PBS.