| Astragalus sagitticarpus A.E. Estrada, Villarreal & Encina, 2020 |
| Astragalus saltonis M.E. Jones, 1923 |
| Astragalus sanctorum Barneby, 1976 |
| Astragalus sanguineus Rydb., 1929 |
| Astragalus scalaris S. Watson, 1888 |
| Astragalus scalaris var. quercetinus M.E. Jones, 1923 |
| Astragalus schaffneri M.E. Jones, 1923 |
| Astragalus scutaneus Barneby, 1956 |
| Astragalus seatonii M.E. Jones, 1895 |
| Astragalus seatonii var. crucis M.E. Jones, 1923 |
| Astragalus selenaeus Greene, 1895 |
| Astragalus sinaloae Barneby, 1964 |
| Astragalus sonorae A. Gray, 1853 |
| Astragalus spellenbergii A.E. Estrada, S. González & Villarreal, 2016 |
| Astragalus stenophyllus Torr. & A. Gray, 1838 |
| Astragalus stenophyllus var. filipes (Torr. & A. Gray) Tidestr., 1937 |
| Astragalus stenophyllus var. residuus (Jeps.) Barneby, 1950 |
| Astragalus stictocarpus (Rydb.) Tidestr., 1937 |
| Astragalus streptopus Greene, 1885 |
| Astragalus strigulosus Kunth, 1823 |