Mimulus ringens is a species of monkeyflower known by the common names Allegheny monkeyflower and square-stemmed monkeyflower. It is native to eastern and central North America, and there are occurrences in the western United States, some of which may represent introductions. It grows in a wide variety of wet habitat types. This is rhizomatous perennial growing 20 centimeters to well over a meter tall, its 4-angled stem usually erect. The oppositely arranged leaves are lance-shaped to oblong, up to 8 centimeters long, and sometimes joined or nearly so clasping the stem. The herbage is hairless. The flower 2 to 3 centimeters long, its tubular base encapsulated in a ribbed calyx of sepals with pointed lobes. The flower is lavender in color and divided into an upper lip and a larger, swollen lower lip. One variety of this plant, var. colophilus, is rare, ecologically restricted, and vulnerable. It is known from Quebec, it has been reported in Vermont, and there are a few occurrences in Maine, where it grows only in freshwater sections of tidal estuaries. This plant variety faces several threats, but its current status is not known due to a lack of data.