Purpose <p>This study aimed to retrospectively characterize patients with biochemical failure and peritoneal metastases detected by PSMA PET/CT, describe the detectability of ¹⁸F-FDG PET/CT and conventional imaging; and assess patient outcome.</p> Methods <p>We searched an institutional database for patients who underwent ⁶⁸Ga-PSMA-11 or ¹⁸&#xa0;F-DCFPyL PET/CT between 5/2016 and 9/2024 and measured the SUVmax of the most avid peritoneal lesion. We assessed correlative CT, MRI, or FDG PET/CT performed within 90 days and validated peritoneal metastases via composite reference. Survival was estimated with Kaplan-Meier analysis. Seven patients subsequently treated with ¹⁷⁷Lu-PSMA-617 were analyzed.</p> Results <p>Of 9,877 screened patients, peritoneal metastases were confirmed in 57. Most had Gleason ≥ 4 + 3 (81%) and median PSA was 10.0 ng/ml (range 0.34–1,304); 54% were castration-sensitive. Isolated peritoneal disease occurred in 19 (33%). The median SUVmax was 8.2. Among 29 patients with correlative CT/MRI, 41% were negative. Of 14 patients with correlative FDG PET/CT; 43% demonstrated FDG-avid peritoneal disease. On follow-up (<i>n</i> = 42), peritoneal disease improved in 38%. Of those with isolated peritoneal metastases and follow-up data (<i>n</i> = 12), 58% improved. Median overall survival from detection was 53 months (95% CI: 29–not reached), and associated with lower PSA, fewer peritoneal lesions, and non-castration resistance. All seven patients treated with ¹⁷⁷Lu-PSMA-617 progressed.</p> Conclusion <p>Peritoneal metastases from prostate cancer are rare. Nevertheless, PSMA-avid peritoneal lesions may represent metastatic spread from prostate cancer, even in the absence of systemic disease. FDG PET/CT and CT/MRI often miss peritoneal metastases. Peritoneal disease does not necessarily indicate poor prognosis, particularly when isolated.</p> Graphical abstract <p></p>