Pembrolizumab is standard adjuvant therapy for high-risk clear cell renal cell carcinoma, but serositis is an uncommon immune-related adverse event that may mimic recurrence or infection. We report a 55-year-old man who achieved no evidence of disease after nephrectomy and metastasectomy and developed anasarca, large bilateral pleural effusions, mild ascites, peripheral eosinophilia, and a small pericardial effusion after six cycles of adjuvant pembrolizumab. Pleural fluid was exudative and contained 20% eosinophils. Cytology showed inflammatory cells without evidence of malignancy; bacterial, mycobacterial, and fungal studies were negative; and mildly elevated adenosine deaminase did not support tuberculosis. Cardiac function and natriuretic peptides were preserved. Pembrolizumab was discontinued, thoracentesis and corticosteroids were administered, and symptoms, eosinophilia, renal function, and albumin improved rapidly. Follow-up through March 2026 showed no oncologic progression, although some residual pleural and abdominal fluid persisted alongside imaging findings suggestive of portal-hypertension physiology, which may have contributed to residual fluid but did not explain the eosinophilic pleural syndrome. In a targeted literature review, effusion eosinophil data were infrequently reported. This case highlights a likely underrecognized eosinophilic pleural-fluid phenotype within pembrolizumab-associated polyserositis and supports routine differential cell counts in drained serosal fluid when immune-related serositis is suspected.