• Williamsonia Davis, 1913 [Order: Odonata]
• Williamsonia P. Lefébure, 1947 (uncertain) [Order: Bacillariophyta (awaiting allocation)]
• Williamsonia W. Carruthers, 1870 † [Order: Cycadeoideales]
• Williamsonia fletcheri Williamson, 1923
• Williamsonia gigas (Lindley & Hutton) †
• Williamsonia lintneri (Hagen in Selys, 1878)
• Williamsonia pecten †
• Williamsoniaceae (Carruthers, 1870) Nathorst, 1913 †
• Williamsonianthus R. Kräusel & F. Schaarschmidt, 1966 † (uncertain)