The Marsh Speedwell has long-stalked flowers borne in open racemes from the upper leaf axils. Each flower is about 1 cm across with 4 blue, egg-shaped lobes. Lobes are fused at the base forming a short tube, greenish on the inside of the throat, with 2 white stamens and a single style projecting from the center. Only a few flowers may be open at any given time. The leaves are long and narrow, up to 7 cm long and about 1 cm wide. Stems and leaves are typically hairless, occasionally sparsely hairy, rarely quite hairy.
A circumpolar species of both eastern and western hemispheres, it is widespread throughout the upper part of North America, preferring wetter locations at higher elevations in its southern range and extending north beyond the Arctic Circle.
The small, wispy stature and sparse floral array make Marsh Speedwell a very inconspicuous species in its swampy habitats.