Primary rhabdomyosarcoma (RMS) of the adult prostate is a very rare tumor with only 45 cases published to date. The clinical course of RMS of the prostate is very aggressive, and prognosis is very poor. Here we describe two cases of primary RMS of the prostate of adult patients and discuss the differential diagnosis of RMS with other mesenchymal tumors of prostate. The first patient was a 50-year-old man who clinically presented with urinary retention and hematuria, low serum prostate-specific antigen, moderate prostatomegaly, and multiple metastases as shown by computed tomography. Histological examination revealed a diffuse “small round blue cell” proliferation with an intermingled population of larger cells with rhabdomyoblastic differentiation that showed immunoreactivity for desmin and MYF4 (myogenin). Molecular and cytogenetic studies did not reveal recurrent chromosomal translocations associated with RMSs and other sarcomas. The patient underwent doxorubicin (adriamycin) treatment without disease progression during the whole follow-up period of 7 months. The second patient was a 39-year-old man who presented with urinary retention, an increased serum prostate-specific antigen level, extensive prostatomegaly, enlarged abdominal and inguinal lymph nodes, and skeletal metastases. Histological examination revealed a tumoral proliferation with a small round blue aspect and diffuse immunoreactivity for desmin and MYF4 (myogenin). Fluorescence in situ hybridization revealed a FOXO1 translocation, consistent with a diagnosis of alveolar-type RMS. The patient followed a multimodal treatment and died of the disease after its progression.