A 17-year-old patient presented to the emergency department of the General Hospital of Trikala, Greece, reporting hypogastric pain accompanied by a fever of up to 38°C. The pain, progressively increasing in intensity, had been present for about a week, with the fever onset occurring 24 hours prior. Based on clinical examination, transvaginal ultrasound, and computed tomography findings, an incorrect diagnosis of pyosalpinx was made, and intravenous treatment with broad-spectrum antibiotics was initiated. However, the lack of improvement in the patient's clinical and laboratory findings after two days led to the decision to perform a laparotomy. Intraoperatively, a periappendiceal abscess was found, with a bilateral secondary extension of inflammation to the uterus, fallopian tubes, ovaries, and pelvic peritoneum. The inflamed appendix was resected from its retrocecal position, and the pelvic abscess was drained. The postoperative course was uneventful. This case report highlights an atypical presentation of acute appendicitis with abscess, which was preoperatively misdiagnosed as pyosalpinx. The main factors contributing to the misdiagnosis of acute appendicitis and the subsequent delay in medical care are discussed, emphasizing that early and accurate diagnosis is crucial in preventing adverse outcomes and ensuring effective treatment.