This report discusses a rare case of adenomyosis in a noncommunicating functional rudimentary horn, initially described as a subserous myoma on imaging. A 35-year-old nulligravid presented with dysmenorrhea since menarche, acute right lower quadrant pain, and a palpable right pelvic mass. A myomectomy was initially planned, but during the laparoscopic procedure, a rudimentary horn adjacent to a unicornuate uterus was discovered. The patient underwent a second procedure for diagnostic hysteroscopy, diaphonoscopy, chromotubation, laparoscopic excision of the rudimentary horn, and right salpingectomy, recovering without complications. Histopathology revealed diffuse adenomyosis with proliferative endometrium. The case emphasizes the challenges in diagnosing such conditions through imaging. It highlights the importance of considering Müllerian anomalies in pelvic mass diagnoses, particularly when atypical symptoms or imaging findings are present.