| Astragalus orizabae var. irolanus M.E. Jones, 1923 |
| Astragalus orthanthus A. Gray, 1864 |
| Astragalus oxyphysopsis Barneby, 1976 |
| Astragalus oxyphysus A. Gray, 1864 |
| Astragalus oxyrhynchus Hemsl., 1879 |
| Astragalus painteri Rose ex Jones, 1923 |
| Astragalus palmeri A. Gray, 1868 |
| Astragalus parishii A. Gray, 1883 |
| Astragalus parvus Hemsl., 1879 |
| Astragalus peirsonii Munz & McBurney, 1932 |
| Astragalus pennellianus Barneby, 1964 |
| Astragalus peonis (M.E. Jones) James, 1951 |
| Astragalus pervelutinus Rydb., 1929 |
| Astragalus pilosior Spellenb. E.& W. Anderson, 2019 |
| Astragalus piscinus (M.E. Jones) Barneby, 1964 |
| Astragalus playanus M.E. Jones, 1898 |
| Astragalus pomonensis M.E. Jones, 1902 |
| Astragalus pomphocalyx Villarreal & M.A. Carranza, 1994 |
| Astragalus potosinus Barneby, 1964 |
| Astragalus pringlei S. Watson, 1886 |