Two-dimensional boron nitride nanosheets (BNNSs) hold great promise as thermal management materials because of their ultrahigh thermal conductivity and wide band gap. However, the scalable exfoliation of hexagonal boron nitride (h-BN) into few-layered BNNSs remains a challenge. Herein, we proposed a novel tannic acid (TA)-assisted liquid-phase exfoliation approach to realize efficient exfoliation and functionalization of h-BN in an aqueous medium. This method gave rise to a high exfoliation yield of 42.2% and the resultant TA-functionalized BNNSs (BNNSs@TA) showed good dispersion in both water and organic liquids. Additionally, the BNNSs@TA can easily combine with poly(vinyl alcohol) (PVA) to give flexible free-standing composite films with an ultrahigh in-plane thermal conductivity of 70.3 W m(-1) K-1 because of the enhanced intermolecular hydrogen bonds between the attached TA and PVA chains. This study provides a simple, environmentally friendly, and highly efficient approach to achieving the exfoliation of BNNSs and highlights the critical role of BNNS surface functionalization in determining the thermal conductivity of composite films.