We propose a new constraint on light (sub-GeV) particles beyond the Standard Model that can be produced inside the protoneutron star core resulting from the core-collapse supernova explosion. It is derived by demanding that the energy carried by exotic particles being transferred to the progenitor stellar envelopes not exceed the explosion energy of ≲ 2 ×1051 erg of observed supernovae. We show specifically that for the case of a dark photon which kinetically mixes with the SM photon and decays predominantly to an e± pair, a smaller mixing parameter of 1 order of magnitude below the well-established supernova cooling bound can be excluded. Furthermore, our bound fills the gap between the cooling bound and the region constrained by (non)observation of γ rays produced from supernovae for dark photons lighter than ̃ 20 MeV . Our result also rules out the possibility of aiding successful supernova explosions by transferring energy from the supernova core to the shock with exotic particles.