We present the case of a 79-year-old man with an irreducible and painful left groin mass. Obtaining a detailed history of present illness and assessment of radiological findings revealed an abscess in a groin hernia sac, which was formed secondary to perforation of the descending colon. Various examinations, however, failed to reveal the cause of the bowel perforation. Percutaneous drainage improved his symptoms. He was discharged home 24 days after admission with no further sequelae. An abscess in a groin hernia sac is very rare. Although neither bowel perforation nor hernia is rare, surgeons do not sufficiently consider the possibility of an abscess in a groin hernia sac. Thus, surgeons often encounter unexpected abscesses in the groin hernia sac during operations. Our case points to the necessity of obtaining a detailed history of present illness as well as assessment of radiological findings, and presents a therapeutic option for an abscess in a groin hernia sac.