Background: Novel perioperative strategies are needed to reduce recurrence rates in patients undergoing nephrectomy for high-risk, non-metastatic clear cell renal cell carcinoma (ccRCC). We sought to (1) elucidate the effects of PD-1 inhibition on primary tumor-infiltrating and circulating immune cell populations in ccRCC and (2) correlate tumor microenvironment and circulating immune cell compositions with response to anti-PD-1 therapy. Methods: We conducted a prospective, phase I trial of neoadjuvant nivolumab prior to nephrectomy in 15 evaluable patients with non-metastatic ccRCC. We leveraged tissue from this cohort to elucidate the effects of PD-1 inhibition on immune cell populations in ccRCC and correlate the evolving immune milieu with anti-PD-1 response using fluorescence-activated cell sorting, bulk RNA sequencing with protein activity inference, and enzyme-linked immunosorbent assay for circulating cytokines. Results: We found that nivolumab durably promotes a pro-inflammatory state within the primary tumor, as evidenced by a sustained increase in the effector T cell phenotype and decreased representation of regulatory T cell subsets. Baseline immune infiltration within the primary tumor including T effector and myeloid enrichment along with angiogenic depletion correlates with nivolumab responsiveness. Nivolumab increases CTLA-4 expression in the primary tumor, and subsequent nephrectomy increases circulating concentrations of sPD-L1, sPD-L3 (sB7-H3), and s4-1BB. Conclusions: Our findings form the basis to consider neoadjuvant immune checkpoint inhibition (ICI) for high-risk ccRCC while the tumor remains in situ and provide the rationale for perioperative strategies of novel ICI combinations. Clinical trial information: NCT02575222.