Romneya trichocalyx is species of flowering plant in the poppy family. This poppy is native to southern California and Baja California, where it grows in dry canyons in chaparral and coastal sage scrub plant communities, sometimes in areas recently burned.It is a popular ornamental plant, kept for its large, showy flowers. This is a shrub which may exceed two meters in height, its woody stem growing from a network of rhizomes. The gray-green, waxy-textured leaves are each divided into a few lance-shaped lobes, the blades growing up to 20 centimeters long. The inflorescence is a large, solitary flower with six crinkly white petals each up to 10 centimeters long. At the center of the flower is a cluster of many yellow stamens. The fruit is a bristly capsule 3 or 4 centimeters long containing many tiny seeds. This plant bears the largest flowers of any species native to California, rivaled only by Hibiscus lasiocarpos. It was nominated for the honor of California state flower in 1890, but the California poppy won the title in a landslide.

This plant is very similar to Romney coulteri in both appearance and requirements. This species differs only in having more prominent bristles on the sepals. Most plants sold in nurseries in California will be coulteri; trichocalyx is rarely available. The two species will hybridize and there is at least one named horticultural hybrid known as 'White Cloud'.