Halophytic grasses have been recently targeted as possible sources of nutraceutical and medicinal compounds. Nonetheless, few studies have been conducted on the phytochemistry and biological activities of metabolites produced by these plants. Among these, Spartina maritima (Curtis) Fernald, Spartina patens (Aiton.) Muhl., and Puccinellia maritima (Hudson) Parl. are three halophytic grasses whose chemical composition and bioactivities are unknown. The present work broadens the knowledge on the polyphenolic and chlorophyll composition of these species identifying for the first time hydroxycinnamic acids and their derivatives, flavones, flavonols, lignans, as well as chlorophylls and xantophylls. The extracts were particularly rich in caffeic and ferulic acids as well as in trihydroxymethoxyflavone, apigenin and tricin derivatives. Interestingly, several of the identified compounds are relevant from a medicinal and nutraceutical point of view putting in evidence the potential of these species. Thus, the antioxidant, anti-acetylcholinesterase, antibacterial, and antifungal activities of the polyphenolic extracts were assessed as well as the photophysical properties of the chlorophyll-rich extracts. The results, herein presented for the first time, reinforce the nutritional and the medicinal potential of these halophytic grasses.