Natural killer (NK) cells, which are key components of the innate immune response, are crucial for ensuring the efficacy of vaccines as they rapidly eliminate infected cells and enhance the adaptive immune response, ensuring robust and lasting protection. In this report, we investigated the effect of <i>Cladophora wrightiana</i> var. <i>minor</i> (CW) extract, a marine alga, in activating NK cells, as an adjuvant to inactivated A/Puerto Rico/8/34 H1N1 influenza vaccine (iPR8). In vitro, CW extract significantly enhanced the level of activation markers CD69 and CD107a on NK cells and triggered intracellular secretion of interferon gamma (IFN-γ) and granzyme B (GrB), indicating effective NK cell stimulation and cytotoxic function. In vivo, CW extract promoted substantial NK cell recruitment and activation, resulting in higher NK cell populations and elevated post-immunization levels of activation markers. Additionally, CW extract increased IFN-γ and GrB production in CD8⁺ T cells, highlighting its broader impact on the immune response. We also found direct evidence that CW-activated NK cells and dendritic cells (DCs) interacted with and induced the activation of immature DCs and resting NK cells, respectively. These findings suggest that CW extract is a promising adjuvant for nasal vaccines, enhancing cellular immunity by activating NK cells and supporting interactions with DCs and CD8⁺ T cells.