Two-dimensional or ultrathin layered materials are attracting broad interest in both fundamental science and applications. While exfoliation can provide high-quality single- and few-layer flakes with nanometer to micrometer size, the development of wafer-scale synthesis methods is important for realizing the full potential of ultrathin layered materials. Here we demonstrate the growth of high quality few-layer boron nitride (BN) films with controlled thickness by magnetron sputtering of B in N(2)/Ar, a scalable process using only benign, nontoxic reagents. BN films up to two atomic layers are synthesized by reactive deposition at high substrate temperatures. Thicker monocrystalline BN films with an arbitrary number of atomic layers are achieved in a two-step process comprising cycles of alternating room temperature deposition and annealing. Tunneling transport across these BN films shows pinhole-free insulating behavior on μm(2) scales, demonstrating the realization of high quality ultrathin dielectrics.