The marine environment represents one of the most chemically diverse and biologically rich ecosystems on Earth, harbouring an extraordinary repertoire of organisms that have evolved unique biochemical adaptations in response to complex ecological pressures. As terrestrial plant resources face increasing constraints from overharvesting, habitat loss, and diminishing chemical novelty, the marine realm has emerged as a transformative frontier for pharmaceutical discovery. This review examines marine plants, including microalgae, macroalgae (seaweeds), and seagrasses, as prolific and largely underexplored sources of bioactive compounds with significant therapeutic potential. The primary objective of this article is to provide a critical and comprehensive evaluation of the pharmacological diversity inherent to marine plant-derived compounds, with particular attention to polysaccharides, polyphenols, terpenoids, alkaloids, and photosynthetic pigments. These compound classes have demonstrated a broad spectrum of biological activities encompassing antioxidant, anti-inflammatory, antimicrobial, antiviral, and anticancer effects. Additionally, this review addresses contemporary advances in extraction and isolation technologies tailored to the physicochemical challenges unique to marine matrices, including high salinity, complex polysaccharide matrices, and compound instability. Preclinical and early-phase translational research are critically evaluated alongside industrial scalability, regulatory pathways, and sustainability concerns associated with marine bioprospecting. The review concludes by identifying critical knowledge gaps and proposing future research directions, with the aim of accelerating the translation of marine plant bioactives into clinically validated therapeutics. Marine plants represent a scientifically compelling and strategically important resource for next-generation drug discovery.