The effect of H/F chemical decoration on the spin switch of a single 3d transition-metal (TM = Mn, Fe, Co) doped boron nitride (BN) sheet is systematically studied using density functional theory plus Hubbard U (DFT+U). It is found that the ground spin state of a TM embedded in a BN sheet is sensitive to the value of the on-site Coulomb energy. Interestingly, we find that the spin of the Fe–BN system is switched from “spin ON (S = 5/2)” to “spin OFF (S = 0)” for H decoration and from “spin high (S = 2)” to “spin low (S = 1/2)” for H-decorated Mn–BN and F-decorated Co–BN systems. Such spin state switching can open a new route to realize the applications of TM-doped BN for spintronics and quantum information.