Each flower is bilaterally symmetrical, with five 20 to 38 millimeter long petals fused into a corolla tube, and four stamens.

The fruit is a round 4 to 6 millimeter long dry capsule that splits open when ripe.

It has been recorded in Alabama, Arkansas, Connecticut, Washington, D.C., Delaware, Florida, Georgia, Iowa, Illinois, Indiana, Kansas, Kentucky, Louisiana, Massachusetts, Maryland, Michigan, Minnesota, Missouri, Mississippi, North Carolina, Nebraska, New Hampshire, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, South Carolina, Tennessee, Texas, Virginia, Wisconsin, and West Virginia.

Like other members of the genus Agalinis, this species is hemiparasitic on a variety of hosts, particularly graminoids.

[8] Agalinis purpurea uses haustoria to connect its roots with those of its host plants, but it also has green tissues, and performs photosynthesis.