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Secondary iliopsoas abscess arising from colorectal cancer is uncommon and diagnostically challenging. We report a woman in her 50s with locally recurrent sigmoid colon cancer who developed a massive gas-forming left iliopsoas abscess during capecitabine, oxaliplatin and bevacizumab. The acute presentation was severe, with altered consciousness reported before ambulance transfer, marked inflammatory response and need for urgent CT-guided drainage and intensive monitoring. Previous Escherichia coli bacteraemia, bilateral ureteral stents, polymicrobial anaerobic abscess flora and bevacizumab exposure created several plausible infection mechanisms. Abscess culture supported an enteric source whereas urine culture yielded discordant organisms. She improved after source control, broad-spectrum antimicrobial therapy with subsequent de-escalation and prolonged drainage, and was discharged after resuming systemic therapy without bevacizumab. This case emphasises early CT, multidisciplinary decision-making and cautious attribution of causality in oncology patients with flank pain and systemic inflammation.