We report the observation of substitutional silicon atoms in single-layer hexagonal boron nitride (h-BN) using aberration corrected scanning transmission electron microscopy. The images reveal silicon atoms exclusively filling boron vacancies. Density functional theory is used to study the energetics, structure, and properties of the experimentally observed structure. The formation energies reveal Si B + 1 as the most stable configuration. In this case, the silicon atom elevates by 0.66 Å out of the lattice with unoccupied defect levels in the electronic bandgap above the Fermi level. Our results unequivocally show that heteroatoms can be incorporated into the h-BN lattice opening the way for applications ranging from single-atom catalysis to atomically precise magnetic structures.