The ability of the peritoneum to undergo metaplasia, especially to Müllerian-type epithelium has been recognized for a long time. The ability of mesothelial cells to modulate cytoskeletal filaments and shape is provided through the distinct form of subserosal cells, called multipotential subserosal cells, that have the function of replicative cells that differentiate into surface epithelium when affected by various stimuli. Rare forms of peritoneal metaplasia include squamous and cartilaginous metaplasia. This report presents a case of fully developed, mature peritoneal squamous metaplasia in an 85-year-old woman who underwent surgery because of small bowel perforation leading to diffuse peritonitis. This rare incidental finding almost always occurs in the background of chronic peritoneal irritation as a response to mesothelial injury. Squamos peritoneal metaplasia should not be interpreted as metastatic squamous cell carcinoma. The most important distinguishing criteria are the lack of nuclear atypia, mitoses, necrosis, and desmoplastic stromal response.