| Willoughbya scandens var. orinocensis (Kunth) Kuntze, 1891 |
| Willoughbya vitifolia (DC.) Kuntze, 1891 |
| Wirtgenia Sch. Bip., 1842 |
| Wirtgenia mexicana Sch. Bip., 1856 |
| Wirtgenia texana Sch. Bip., 1856 |
| Wuerschmittia Schultz-Bip. ex Walp., 1846 |
| Wulffia Neck. ex Cass., 1823 |
| Wulffia angustifolia (A. Rich.) M. Gómez, 1890 |
| Wyethia Nutt., 1834 |
| Wyethia mexicana S. Watson, 1890 |
| Wyethia ovata Torr. & A. Gray, 1848 |
| Xanthidium Delpino, 1871 |
| Xanthidium ambrosioides (Cav.) Delpino, 1871 |
| Xanthisma DC., 1836 |
| Xanthisma arenarium (Benth.) D.R. Morgan & R.L. Hartm., 2003 |
| Xanthisma crutchfieldii (B.L. Turner) D.R. Morgan & R.L. Hartm., 2003 |
| Xanthisma gracile (Nutt.) D.R. Morgan & R.L. Hartm., 2003 |
| Xanthisma gymnocephalum (DC.) D.R. Morgan & R.L. Hartm., 2003 |
| Xanthisma gypsophilum (B.L. Turner) D.R. Morgan & R.L. Hartm., 2003 |
| Xanthisma johnstonii (S.F. Blake) D.R. Morgan & R.L. Hartm., 2003 |