With the aging global population, understanding the role of blue space (BS) in supporting older adults’ mental health is increasingly important. This systematic review synthesizes quantitative and qualitative evidence to examine how BS influences mental health and well-being in this population. Following PRISMA guidelines, we searched Web of Science, Scopus, PubMed, and PsycINFO for studies published between 2004 and 2024. This review protocol was preregistered on PROSPERO (registration number CRD420250651254). Studies examining BS exposure characteristics and mental health outcomes among adults aged 50 and older were included. A total of twenty-three studies (seventeen quantitative, six qualitative) were reviewed. Quantitative findings indicated generally positive associations between BS proximity, quantity, and improved mental health outcomes, while qualitative findings highlighted the therapeutic benefits of natural features, social interactions, and sensory experiences, along with barriers such as accessibility and safety concerns. BS holds significant potential for promoting older adults’ well-being. These findings highlight the potential of BS as a sustainable urban health resource, offering evidence to support integrated planning strategies that promote environmental, public health, and broader sustainability goals. Future research should investigate specific BS characteristics using longitudinal and experimental designs to enhance causal understanding and inform urban planning and public health strategies.