Pristine stars with masses between ∼140 and 260 M⊙ are theoretically predicted to die as pair-instability supernovae. These very massive progenitors could come from Population III (Pop III) stars in the early universe. We model the light curves and spectra of pair-instability supernovae over a range of masses and envelope structures. At redshifts of reionization z≥ 6, we calculate the rates and detectability of pair-instability and core-collapse supernovae, and show that with the James Webb Space Telescope it is possible to determine the contribution of Pop III and Pop II stars towards reionization by constraining the stellar initial mass function at that epoch using these supernovae. We also find the rates of Type Ia supernovae, and show that they are not rare during reionization, and can be used to probe the mass function at 4–8 M⊙. If the budget of ionizing photons was dominated by contributions from top-heavy Pop III stars, we predict that the bright end of the galaxy luminosity function will be contaminated by pair-instability supernovae.