Guaiacum (), sometimes spelled Guajacum, is a genus of flowering plants in the caltrop family Zygophyllaceae. It contains five species of slow-growing shrubs and trees, reaching a height of approximately 20 m (66 ft) but usually less than half of that. All are native to subtropical and tropical regions of the Americas and are commonly known as lignum-vitae, guayacán (Spanish), or gaïac (French). The genus name originated in Taíno, the language spoken by the native Taínos of the Bahamas; it was adopted into English in 1533, the first word in that language of American origin.

Members of the genus have a variety of uses, including as lumber, for medicinal purposes, and as ornamentals. The trade of all species of Guaiacum is controlled under CITES Appendix II.

Guaiacum officinale is the national flower of Jamaica, while Guaiacum sanctum is the national tree of the Bahamas.