The weak photoluminescence of spin defects in hexagonal boron nitride limits their use in quantum technologies. Here, the authors demonstrate that a van der Waals heterostructure enhances defect emission by up to 45-fold at 4 K through interlayer optical coupling, with evidence supporting nonlinear photon-mediated excitation and interlayer energy-transfer pathways, while preserving magnetic-field readout and room-temperature operation.