Neuroendocrine prostate cancer (NEPC) is an aggressive subtype of castrate-resistant prostate cancer characterized by poor patient outcome. Whole transcriptome sequencing analyses identified a NEPC-specific RNA splicing program that is predominantly controlled by the SRRM4 gene, suggesting that SRRM4 drives NEPC development. However, whether SRRM4 expression in patients may aid pathologists in diagnosing NEPC and predicting patient survival remains to be determined. In this study, we have applied RNA in situ hybridization and immunohistochemistry assays to measure the expressions of SRRM4, NEPC markers (SYP, CD56, and CHGA), and adenocarcinoma (AdPC) markers (AR, PSA) in a series of tissue microarrays constructed from castrate-resistant prostate tumors, treatment-naïve tumors collected from radical prostatectomy, and tumors treated with neoadjuvant hormonal therapy (NHT) for 0-12 months. Three pathologists also independently evaluated tumor histology and NEPC marker status. Here, we report that SRRM4 in castrate-resistant tumors is highly expressed in NEPC, strongly correlated with SYP, CD56, and CHGA expressions (Pearson correlation r = 0.883, 0.675, and 0.881; P < 0.0001) and negatively correlated with AR and PSA expressions (Pearson correlation r = -0.544 and -0.310; P < 0.05). Overall survival is 12.3 months for patients with SRRM4 positive tumors, comparing to 23 months for patients with SRRM4 negative tumors. In treatment-naïve AdPC, low SRRM4 expression is detected in ∼16% tumor cores. It correlates with SYP and CHGA expressions, but not Gleason scores. AdPC treated with >7 month NHT has significantly higher SRRM4 expression. Based on these findings, we conclude that SRRM4 expression in castrate-resistant tumors is highly correlated with NEPC and poor patient survival. It may serve as a diagnosis and prognosis biomarker of NEPC.