Anaplastic lymphoma kinase (ALK) is a tyrosine kinase with genomic and expression changes in many solid tumors. ALK inhibition is the first-line therapy for lung cancers with ALK alterations, and an effective therapy in other tumor types, but has not been well-studied in prostate cancer. Here, we aim to delineate the role of ALK genomic and expression changes in primary and metastatic prostate cancer. We determined ALK expression by IHC and RNA sequencing, and genomic alterations by NGS. We assessed functional consequences of ALK overexpression and pharmacologic ALK inhibition by cell proliferation and cell viability assays. Among 372 primary prostate cancer cases, we identified one case with uniformly high ALK protein expression. Genomic analysis revealed a novel SLC45A3-ALK fusion which promoted oncogenesis in in vitro assays. We observed ALK protein expression in 5 of 52 (9%) of metastatic prostate cancer cases, of which 4 of 5 had neuroendocrine features. ALK-expressing neuroendocrine prostate cancer had a distinct transcriptional program, and earlier disease progression. An ALK-expressing neuroendocrine prostate cancer model was sensitive to pharmacologic ALK inhibition. In summary, we found that ALK overexpression is rare in primary prostate cancer, but more frequent in metastatic prostate cancers with neuroendocrine differentiation. Furthermore, ALK fusions similar to lung cancer are an occasional driver in prostate cancer. Our data suggest that ALK-directed therapies could be an option in selected patients with advanced prostate cancer.Significance:Anaplastic lymphoma kinase (ALK) is a validated drug target in cancer. Here we delineate the spectrum of ALK alterations in prostate cancer. We show that ALK overexpression is present in advanced prostate cancers, in particular in cases with features of neuroendocrine carcinoma. Furthermore, ALK expression is associated with responses to pharmacologic ALK inhibition. Our study demonstrates that ALK-directed therapies should be considered in selected prostate cancer cases.