The sea louse Caligus rogercresseyi is a major ectoparasitic copepod that causes significant economic losses in the salmon farming industry. Despite recent advancements, the mechanisms underlying germline and embryo development in this species remain poorly understood. The Vasa gene encodes a highly conserved DEAD box helicase that is required for germ cell formation and function in many species. In this study, the Vasa gene was characterized in C. rogercresseyi, and its expression and function were analyzed. Phylogenetic analysis showed that the Cr-Vasa gene product formed clusters in clades with Vasa proteins from closely related species of crustaceans. Cr-Vasa gene expression patterns were assessed by qPCR, and the results showed a significantly higher relative expression level in adult females compared to copepodid, chalimus, and adult male stages. Tissue-specific localization of Cr-Vasa mRNA in C. rogercresseyi was determined using chromogenic in situ hybridization, and strong positive signal was observed in male testes, but also in the intestine and cuticle, while in females, it was observed in the ovaries, oocytes, cuticle, intestine, and egg strings. RNAi-mediated gene silencing of Cr-Vasa impacted embryonic development and reproductive output in adult female lice. Females from the dsVasa-treated group displayed unusual phenotypes, including shorter egg strings with numerous extra-embryonic inclusions, irregularly shaped abnormal embryos, and aborted egg strings. This study provides insights into the role of the Vasa gene in C. rogercresseyi embryonic development and reproductive output, which may have implications for the control of this parasitic copepod in the salmon farming industry.