Immune checkpoint inhibitor monotherapy in metastatic castration-resistant prostate cancer (mCRPC) has produced modest results. High-dose radiotherapy may be synergistic with checkpoint inhibitors. To evaluate the efficacy and safety of the PD-L1 inhibitor avelumab with stereotactic ablative body radiotherapy (SABR) in mCRPC. From November 2017 to July 2019, this prospective phase 2 study enrolled 31 men with progressive mCRPC after at least one prior androgen receptor–directed therapy. Median follow-up was 18.0 mo. Avelumab 10 mg/kg intravenously every 2 wk for 24 wk (12 cycles). A single fraction of SABR (20 Gy) was administered to one or two disease sites within 5 d before the first and second avelumab treatments. The primary endpoint was the disease control rate (DCR), defined as a confirmed complete or partial response of any duration, or stable disease/non–complete response/non–progressive disease for ≥6 mo (Prostate Cancer Clinical Trials Working Group 3–modified Response Evaluation Criteria in Solid Tumours version 1.1). Secondary endpoints were the objective response rate (ORR), radiographic progression-free survival (rPFS), overall survival (OS), and safety. DCR and ORR were calculated using the Clopper-Pearson exact binomial method. Thirty-one evaluable men were enrolled (median age 71 yr, 71% with ≥2 prior mCRPC therapy lines, 81% with >5 total metastases). The DCR was 48% (15/31; 95% confidence interval CI 30–67%) and ORR was 31% (five of 16; 95% CI 11–59%). The ORR in nonirradiated lesions was 33% (four of 12; 95% CI 10–65%). Median rPFS was 8.4 mo (95% CI 4.5–not reached NR) and median OS was 14.1 mo (95% CI 8.9–NR). Grade 3–4 treatment-related adverse events occurred in six patients (16%), with three (10%) requiring high-dose corticosteroid therapy. Plasma androgen receptor alterations were associated with lower DCR (22% vs 71%, p = 0.13; Fisher’s exact test). Limitations include the small sample size and the absence of a control arm. Avelumab with SABR demonstrated encouraging activity and acceptable toxicity in treatment-refractory mCRPC. This combination warrants further investigation. In this study of men with advanced and heavily pretreated prostate cancer, combining stereotactic radiotherapy with avelumab immunotherapy was safe and resulted in nearly half of patients experiencing cancer control for 6 months or longer. Stereotactic radiotherapy may potentially improve the effectiveness of immunotherapy in prostate cancer. In this phase 2 trial, avelumab with stereotactic ablative body radiotherapy achieved disease control in approximately half of patients with advanced treatment-refractory metastatic castration-resistant prostate cancer and radiographic responses in nearly one-third of patients, with an acceptable toxicity profile. This combination has promising activity and warrants further investigation in randomised trials.