Male infertility has become an important health problem that is primarily caused by testicular dysfunction with abnormal spermatogenesis. In this study, we demonstrated that the neuropeptide, substance P (SP), is essential for spermatogonia proliferation in a seminiferous tubule culture system. In addition, SP (5 nmol/kg) treatment markedly restored spermatogenesis, improved sperm quality, and increased the number of ZBTB16⁺ or LIN28⁺ undifferentiated spermatogonia as well as STRA8⁺ differentiated spermatogonia in a busulfan-induced non-obstructive azoospermic mouse model. Furthermore, 100 nM SP treatment in vitro significantly stimulated the proliferation of GC-1 spg cells (a spermatogonia cell line) via activation of the Erk1/2 signaling pathway. Moreover, the sperm quality and the number of spermatogonia were significantly reduced after treatment with RP67580, a selective NK-1 receptor antagonist, suggesting that SP-NK1R signaling plays an important role in spermatogenesis. Taken together, these results suggest that SP may be a potential therapeutic agent for male infertility by accelerating the restoration of spermatogenesis.