Barr's Milkvetch

Perennial, Herbs, Stems woody below, or from woody crown or caudex, Taproot present, Nodules present, Stems very short, acaulescent or subacaulescent, Stems less than 1 m tall, Stems solid, Stems or young twigs sparsely to densely hairy, Stems hairs pilose or spreading, Stems silvery, canescent, tomentose, cobwebby, or wooly, Leaves alternate, L eaves petiolate, Stipules conspicuous, Stipules membranous or chartaceous, Stipules persistent, Stipules connate to each other, forming a tuber or sheath, Leaves compound, Leaves palmately 2-3 foliate, Leaves odd pinnate, Leaf or leaflet margins entire, Leaflets opposite, Leaves hairy on one or both surfaces, Flowers in axillary clusters or few-floweredracemes, 2-6 flowers, Inflorescence axillary, Bracts very small, absent or caducous, Flowers zygomorphic, Calyx 5-lobed, Calyx hairy, Petals separate, Corolla papilionaceous, Petals clawed, Petals pinkish to rose, Petals blue, lavander to purple, or violet, Banner petal narrow or oblanceolate, Wing petals narrow, oblanceolate to oblong, Wing tips obtuse or rounded, Keel petals auriculate, spurred, or gibbous, Keel tips obtuse or rounded, not beaked, Stamens 9-10, Stamens diadelphous, 9 united, 1 free, Filaments glabrous, Style terete, Style persistent in fruit, Fruit a legume, Fruit unilocular, Fruit freely dehiscent, Fruit el ongate, straight, Fruit oblong or ellipsoidal, Fruit exserted from calyx, Fruit beaked, Fruit glabrous or glabrate, Fruit hairy, Fruit 1-seeded, Seeds cordiform, mit-shaped, notched at one end, Seed surface smooth, Seeds olive, brown, or black.

Astragalus barrii is a species of flowering plant in the legume family known by the common name Barr's milkvetch. It is native to the United States, where it is a "regional endemic", occurring in parts of southwestern South Dakota, northeastern Wyoming, southeastern Montana, and Nebraska.

This mat-forming perennial herb grows only about 4 inches in height. The leaves are each made up of three lance-shaped leaflets. The herbage is coated in white hairs which are dolabriform, or shaped like the head of an ax. The inflorescence contains iridescent pinkish purple flowers with petals up to 1.7 centimeters long. The fruit is a legume pod up to 8 millimeters in length. Blooming occurs in late April through mid-June; April flowering generally only occurs in years with light snowfall.

This plant occurs on buttes and badlands in the Powder River Basin and surrounding areas. The habitat is generally mixed-grass prairie on limestone, sandstone, shale, or siltstone. It grows alongside other species of Astragalus including A. gilviflorus, A. hyalinus, A. bisulcatus, A. racemosus, and A. spatulatus, and it must be in flower to be distinguished from other species with three leaflets on each leaf.

This plant was first collected in 1900 in Wyoming. It was described as a species in 1956 and named for the botanist and cattle rancher Claude Arno Barr (1887–1982).