We demonstrate a new paradigm in nanoscale thermometry exploiting well-known switching measurements of a superconducting weak link. Such a weak link probed with nanosecond current pulses serves as a temperature-sensing element and, because of the fast inherent dynamics, is capable of delivering unprecedented temporal resolution. We use the thermometer to measure the dynamic temperature of electrons in a long superconducting wire relaxing to the bath temperature after application of a heating pulse, involving evaluation of the retrapping time. Our measurement delivers resolution better than 10 ns, with potential for further improvement. It extends the temporal resolution of existing experiments and introduces new possibilities for investigating thermodynamics at the nanoscale.