Alveolar rhabdomyosarcoma is an uncommon tumor affecting adolescents and young adults that is only rarely encountered in body fluid cytology. We report the cytological features of metastatic alveolar rhabdomyosarcoma in the ascitic fluid of a 17-year-old female patient, who had presented with abdominal distention, 21 months after being diagnosed with perirectal alveolar rhabdomyosarcoma. The rare single neoplastic cells that were admixed with abundant reactive mesothelial cells were initially misinterpreted as reactive mesothelial cells. However, their neoplastic nature was established after a careful review of their cytological features and the performance of immunoperoxidase stains. Compared to the reactive mesothelial cells that were present in the sample, the malignant cells were smaller, with less ample and more homogenous cytoplasm. They had slightly larger, more hyperchromatic, and more frequently eccentric nuclei, with larger nucleoli. This case highlights the potential pitfall of the misinterpretation of metastatic alveolar rhabdomyosarcoma cells for reactive mesothelial cells. Awareness of this potential diagnostic problem and recognition of the cytomorphological features of this neoplasm in the body fluids allows the identification of malignant cells, even when they are rare and intimately associated with mesothelial cells.