M. minuta has a strongly variable appearance, which often leads to confusion with closely related species. In the water the plant is creeping and spreading, while on land it can appear cushion-like. It typically is perennial but sometimes appears annual. It is a tenagophyte, with the juvenile growing submerged and the adult typically terrestrial. It has a light brown to green rhizome that is 0.4–0.8 millimetres (⁄64–⁄32 in) thick with short tan hairs at the ends and internodal roots. The land leaves are on erect, terete, 5–13 centimetres (2.0–5.1 in) long petioles. The leaflets are 0.8–1.7 centimetres (⁄8–⁄8 in) by 1.2–2 centimetres (⁄2–⁄4 in), mostly glabrous, cuneate or flabellate. The leaves in water are typically not floating, but emergent from the water. Fertile leaves are produced on land with up to four sporocarps each at penduncles near the base of the petiole. It has a small sporocarp that is 2.6–4.1 millimetres (⁄64–⁄32 in) long, 2.4–3.1 millimetres (⁄32–⁄8 in) wide, and 1.3–1.7 millimetres (⁄64–⁄16 in) thick. The sporocarp has a superior tooth at the apex of the stalk and an inferior tooth at the base. The sporocarp has a conspicuous 1.5–2.2 millimetres (⁄16–⁄32 in) long raphe, about ⅔ the length of the sporocarp and semi-terete. Unlike a few Marsilea species, M. minuta sporocarps mature above ground.