Aging of the human testis, conjointly with a decline in reproductive function, is still underexplored and the fundamental molecular mechanisms are poorly understood. Testicular peritubular cells (TPCs) are smooth muscle-like cells, building the wall of seminiferous tubules in men. TPCs transport sperm through the seminiferous tubules and are supposed to be important for spermatogenesis by secretion of proteins to the spermatogonial stem cell (SSC) niche. Therefore, senescence of TPC may also contribute to the age-related decline of testicular functions in men. Due to the limited availability of standardized human samples, we established marmoset (C. jacchus) TPCs (MKTPC) as a model for human TPCs (HTPC). To investigate the degree of similarity between HTPCs and MKTPCs, we performed a LC-MS/MS analysis of TPCs from both species and could find a high degree of sequence homology and a considerable correlation of protein abundances between both species. The similarity of their proteomes introduces MKTPCs as a compelling model for the human system. To trigger senescence of MKTPCs, cells of young individuals (n = 5) were repeatedly passaged. After approximately 10 passages, cells stopped dividing and showed hallmarks of cellular senescence. Proteomes and secretomes of passaged and control MKTPCs were then compared using a label-free approach. Specific proteome alterations in passaged MKTPCs included an increased abundance of mitochondrial and a decreased abundance of smooth muscle cell proteins. Furthermore, proteins of the endoplasmic reticulum and proteins related to mRNA regulatory processes were found to be decreased in passaged MKTPCs. These findings suggest that cellular senescence may impair sperm transport through a reduction of contractility. Additionally, the detected alterations in the secretomes of aged MKTPCs imply changes in the protein composition of the SSC niche.