Astragalus osterhoutii is a rare species of milkvetch known by the common name Osterhout milkvetch. It is endemic to Grand County, Colorado, where it is known from a few populations in Middle Park, a mountain basin. There are five or six occurrences. It is a federally listed endangered species.
The plant grows on sagebrush badlands on soils originating from the Niobrara Formation and the Pierre Shale. The gray-brown shale and siltstone clay soils are high in selenium, which the plant concentrates, causing it to develop a foul odor.
This is a perennial herb producing many slender, erect, green stems up to a meter tall. The leaves are made up of several narrow linear leaflets up to 3 centimeters long. The inflorescence bears cream-colored, pealike flowers each over 2 centimeters long. The fruit is a flattened legume pod up to 4 centimeters long.
Threats to the species include flooding and reservoir creation, which have eliminated at least one occurrence. All-terrain vehicle use and oil and gas exploration activities damage the habitat occupied by the plant.