It has become clear in the past few years that young supernovae can become rather powerful radio sources at an early stage of their evolution. Three young supernovae have been detected so far: SN1970g in M101 (ref. 1), SN1979c in M100 (ref. 2) and SN1980k in NGC6946 (refs 3,4). The data on these objects and on older supernova remnants suggest that the radio emission turns on between 1 week and 1 yr after the supernova event, and that the supernova continues to be a radio emitter for at least a few years. About 50-100yr later, when the supernova shell has swept up enough interstellar gas, the remnant may become visible again in the radio domain5. Previous searches for radio emission from young supernovae have been based on optically discovered supernovae. However, with the many detailed maps of the radio continuum emission from galaxies now available, it becomes feasible to find supernovae by comparing radio data alone. We report here the appearance of a radio source in the northern, inner spiral arm of NGC4258. Inspection of existing optical data (ref. 6, and W. L. W. Sargent and C. Kowal, personal communication) confirms that this source is a young supernova, making it the first radio detected supernova event.