Significance This work investigated the in vivo role of Paxbp1, a poorly studied nuclear protein, in regulating adult mouse muscle stem cells (MuSCs). By deleting Paxbp1 in adult mouse quiescent MuSCs, we found that Paxbp1 -null MuSCs were unable to reenter the cell cycle to proliferate upon muscle injury and subsequently underwent apoptosis, resulting in a total failure in injury-induced muscle regeneration. Mechanistically, we found that Paxbp1 controls a late cell-growth checkpoint by targeting mTORC1. Loss of Paxbp1 in MuSCs led to increased levels of reactive oxygen species that in turn triggered p53 activation and induction of multiple p53 target genes, some of which contributed to cell-cycle arrest (e.g., Cdkn1a ), apoptosis (e.g., Apaf1 ), and impaired mTORC1 signaling (e.g., Sesn2 and Ddit4 ).