Mayer–Rokitansky–Küster–Hauser syndrome (MRKH) is one of the causes of primary infertility, affecting about 1 in 4,500–10,000 women. These patients have vaginal stenosis but normal secondary sexual characteristics. In this case, we present a patient with MRKH who also has adenomyosis in rudimentary horns and an endometrioma in the right ovary. The patient has been experiencing cyclical abdominal pain for the last 15 years. There is evidence of endometriotic lesions in the myometrium of the remnant horn. Case description: Here we present a case of primary infertility in a 35-year-old woman who presented with dull aching abdominal pain for the last 15 years. She was frequently misdiagnosed with a fibroid in the right lateral uterine wall and a left ovarian endometrioma. The diagnosis was confirmed through MRI pelvis and histopathology of the specimen. The patient underwent surgery for the same and has been symptom-free since then. She is now planning for a surrogate child. Conclusion: Based on the imaging findings, intraoperative findings, and histopathological evaluation, it was concluded that this was a rare case of MRKH with functioning endometrium in the remnant horn. Adenomyosis can be explained by the unusual developmental anatomy of the uterus in patients with MRKH syndrome, which allows the endometrial cells to be misplaced within the myometrial layer. The possible explanation for the endometrioma is the theory of coelomic metaplasia as a cause of endometriosis, rather than retrograde menstruation. It is possible to have endometriotic tissue in the remnant horns of the uterus and endometrioma in the ovaries.