Triglochin maritima is a species of flowering plant in the arrowgrass family Juncaginaceae. It is found in brackish marshes, freshwater marshes, wet sandy beaches, fens, damp grassland and bogs. It has a circumboreal distribution, occurring throughout the northern Northern Hemisphere. In the British Isles it is common on the coast, but very rare inland.
It is similar to marsh arrowgrass (Triglochin palustris) but has the following differences: it has stolons, is stouter. The leaves are fleshy and not furrowed above. It is not very aromatic. The raceme are more dense and like sea plantain. The flowers are fleshier<ref name=":0">C. Dwight Marsh, A. B. Clawson, and G. C. Roe Jr (1929). Arrow grass as a Stock-Poisoning Plant. United States Department of Agriculture.</ref>. The fruits are oval, 4 mm long, 2 mm wide.<ref name="The Wild Flower Key">Template:Cite book</ref>
It varies in height from 8-30 inches (to 45 cm). It flowers in May to August; flowers are greenish, 3 petalled, edged with purple, 1/8 inch (2mm) across, in a long spike.<ref>Common Arrow-grass (Triglochin maritima)</ref><ref name="Complete British Wild Flowers">Template:Cite book</ref> Common names include seaside arrowgrass,<ref>Template:PLANTS</ref> common arrowgrass, sea arrowgrass and shore arrowgrass.
Triglochin concinna is a synonym of this species.<ref>Template:Cite web</ref>
This plant is toxic, as it can produce cyanide. This species has been known to cause losses in cattle, with green leaves being more toxic than dried material<ref name=":0" />.