Lyonothamnus is a monotypic genus of trees in the rose family containing the single living species Lyonothamnus floribundus, which is known by the common name Catalina ironwood, and the subspecies ssp. aspleniifolius and ssp. floribundus.

Lyonothamnus is endemic to the Channel Islands of California, where it grows in the chaparral and oak woodlands of the rocky coastal canyons.

This is a tree growing up to 15 metres (49 ft) tall with peeling reddish gray or brown bark. The evergreen leaves are shiny, dark green with lighter undersides, and borne on short petioles. The two subspecies have leaves of different leaf shapes. The inflorescence is a cluster of woolly white flowers with many short, whiskery stamens. The fruit is a pair of hard follicles.

Lyonothamnus is currently a geographically restricted island endemic genus containing one species with two subspecies. Discoveries of fossils have suggested that the genus once comprised several species, all but one now extinct, which grew on the mainland in the southwestern United States.