The monodermal teratoma strona ovarii is a rare ovarian tumour, however, strona ovaril presenting with pseudo-Meigs' syndrome and raised cancer antigen (CA)-125 levels is even rarer. In elderly patients, this presentation can potentially lead to a misdiagnosis of a malignant ovarian carcinoma, resulting in unnecessary extensive surgery. We report a 55-year-old female who presented to the Lok Nayak Jai Prakash Narayan Hospital, New Delhi, India, in 2016 with progressive abdominal discomfort, fatigue and abdominal distention. Clinical and radiological features were indicative of a malignant ovarian tumour and ascites. Serum CA-125 levels were elevated at 258 U/ml. A left-sided salpingo-oophorectomy was performed, after which the serum CA-125 levels normalised. There was no evidence of recurrence at a six-month follow-up. A frozen section procedure confirmed the diagnosis of a struma ovarii. This rare condition should be considered as a differential diagnosis in patients presenting with ovarian masses, ascites and raised CA-125 levels.