Background: Goal of our study is to evaluate the efficacy and safety of immunotherapy in combination with standard salvage radiation therapy (SRT) in patients with biochemical recurrence (BCR) or prostate-specific antigen (PSA) persistence after radical prostatectomy (RP). The combination of immunotherapy (IO) with radiation therapy might provide a clinical benefit to patients with recurrent prostate cancer due to an induction of increased activity of the immune system against cancerous tissue (abscopal effect). Primary endpoint is complete biochemical response (PSA level below detection level) 60 weeks after start of trial treatment, which further defines prognosis of patients. Further explorative endpoints like radiographic progression-free survival, PSA-nadir level and time to PSA-nadir, time to initiation of subsequent therapy (secondary ADT or NHA) and quality of life as well as adverse events will be evaluated. Methods: The trial is as a phase II, open-label, single arm monocenter trial which evaluates the combination of pembrolizumab as study medication in combination with SRT. Pembrolizumab will be administered in a three-weekly scheme up to one year of treatment. A concomitant ADT is administered in high-risk patients only (PSA > 0.7ng/ml or cN+ in imaging studies). All patients will be staged with PSMA PET-CT, if applicable. Additionally, blood and urine samples will be collected for correlative biomarker studies. A total of 49 patients are planned to be enrolled in this explorative study within 2 years. INCLUSION CRITERIA (excerpt) Histologically confirmed diagnosis of an adenocarcinoma of the prostate and a BCR or PSA persistence after RP. Histology of the RP specimen needs to fulfill the following criteria: adenocarcinoma of the prostate, Gleason score 7-10; pNX or pN0 or pN1 (max. 2 lymph nodes involved). Imaging within 30 days prior to study inclusion is mandatory (([68Ga] or [18F] PSMA PET-CT as standard imaging modality, alternatively CT abdomen and full-body bone scan). PSA value between ≥0.2 and ≤1.0 ng/ml, measured at least six weeks postoperatively. EXCLUSION CRITERIA (excerpt) Prior-therapy with an anti-PD-1, anti-PD-L1, or anti PD L2 agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (e.g., CTLA-4, OX 40, CD137). Prior systemic anti-cancer therapy including investigational agents within 4 weeks prior to registration (like neo-adjuvant androgen deprivation therapy (ADT), secondary hormone ablation or taxan-based chemotherapy). Distant metastases or suspicious lymph nodes outside the lower pelvis in imaging with PSMA PET-CT (patients with PET positive bone lesions that are morphologically not clearly suspicious of metastases and would not change clinical practice can be included). Clinical trial information: NCT04931979.