Peer-mediated interventions (PMIs) are evidence-based practices that improve outcomes for students with intellectual and developmental disabilities (IDD). Determining whether peers also benefit substantively from their involvement in these widely used practices is key to establishing the reciprocity of PMIs. This study examined the breadth and depth of ways that peers perceive they are impacted by their experiences in PMIs and the factors that shape this impact. More than 250 secondary and postsecondary peers completed the "Peer-Mediated Impact Survey for Peers" ("PMIS:P"). Peers reported being impacted in multiple positive ways clustered within seven thematic areas: rewarding impact, advocacy impact, changes in views, future impact, social impact, skill and intrapersonal development, and self-worth impact. Several factors were associated with variations in peer impact, including having supported at least one student with autism, supported at least one student who was nonverbal, prior familiarity with a student, received ongoing support from an educator or other school staff, and more time spent together. We provide recommendations for research and practice aimed at addressing the myriad benefits of involving peers in PMIs.