We report a case of acute interstitial nephritis (AIN) in a 68-year-old male with squamous cell carcinoma (SCC) and relapsed/refractory multiple myeloma (RRMM) who developed acute kidney injury (AKI) shortly after starting teclistamab for RRMM. Despite stable renal function on immune checkpoint inhibitor (ICI) therapy for SCC, proton pump inhibitors and trimethoprim-sulfamethoxazole for 3 years, AKI only occurred post-teclistamab initiation. Biopsy-confirmed AIN with serum creatinine improved only after pulse-dose steroids. This case highlights a potential synergistic nephrotoxic effect of teclistamab and ICIs, warranting further investigation into teclistamab's renal safety.