Inner ear hair cells are required for hearing, and their loss is the primary cause of sensorineural hearing loss in human. The prevailing notion is that the mammalian cochlea is unable to spontaneously regenerate lost hair cells, thus leading to irreversible hearing loss. However, it is unknown if the immature cochlea may be more regenerative. Using a transgenic approach, we selectively damaged hair cells and found that hair cells can be regenerated to a modest degree in newborn mice. We tracked the origin of regenerated hair cells and found supporting cells to be their source. In non-mammalian species where hair cells are readily regenerated, progenitor cells first divide before transforming into hair cells. In the newborn mice, we found that supporting cells similarly divide and convert into hair cells. However, regenerated hair cells do not survive, the degree of regeneration is limited, and regeneration is limited to a time window immediately after birth. Together, these findings have helped define a population of progenitor cells in the mammalian cochlea capable of spontaneous hair cell regeneration. Future research on these progenitor cells may reveal cues guiding regeneration to restore hearing.