Marine organisms produce a wide range of secondary metabolites that show the potential for the development of new moieties for the treatment of various diseases. Recently, several studies on secondary metabolites obtained from marine seagrasses resulted in the identification of pharmacologically active constituents. Thalassia, a marine seagrass genus belonging to the Hydrocharitaceae family, consists of two therapeutically useful species, namely, Thalassia testudinum and Thalassia hemprichii. The phytoconstituents present in the Thalassia species showed a variety of chemical compounds with proven therapeutic applications, such as alkaloids (antioxidant, larvicidal activity), glycosides (anticancer, antioxidant, larvicidal activities), fatty acids (anticancer activity), flavone glycosides (Anticancer activity), tannins, phenols (antioxidant, anticancer activities), ketones (antiviral activity), sterols (antioxidant, antiviral activities), flavonoids (anticancer activity), sulphates (antibacterial activity). This review aims to assemble the pharmacological activities of T. testudinum and T. hemprichii of the Thalassia genus, their phytoconstituents, methods for extraction, pharmacological actions, and their mechanisms and IC50 values. Hence, the results highlight the potency of these two species as natural therapeutic molecules for the treatment of diseases.