Eschscholzia californica, the California poppy, golden poppy, Mexican poppy, California sunlight or cup of gold, is a species of flowering plant in the family Papaveraceae, native to the United States and Mexico. It is cultivated as an ornamental plant flowering in summer (spring in southern Australia), with showy flowers in brilliant shades of red, orange and yellow (occasionally pink and white). It is also used as food or a garnish. It had various uses in indigenous herbalism. It became the official state flower of California in 1903.

California poppy is a perennial or annual plant growing to 5–12 in (13–30 cm) tall with alternately branching glaucous blue-green foliage. The leaves are alternately divided into round, lobed segments. The flowers are solitary on long stems, silky-textured, with four petals, each petal 2 to 6 cm (0.79 to 2.36 in) long and broad; flower color ranges through yellow, orange and red. Flowering occurs from February to September in the northern hemisphere. The petals close at night (or in cold, windy weather) and open again the following morning, although they may remain closed in cloudy weather. The fruit is a slender, dehiscent capsule 3 to 9 cm (1.2 to 3.5 in) long, which splits in two, sometimes explosively with an audible snap, to release numerous small 1.5–1.8 mm (0.059–0.071 in) wide black or dark brown seeds. The plant will reseed under ideal conditions, or when winters are cold. Anecdotally, it can be an aggressive reseeder in the home garden context, eventually becoming a groundcover. It survives mild winters in its native range, dying completely in colder climates.

Its native habitat includes California and extends to Oregon, Washington, Nevada, Arizona, New Mexico, Sonora and northwest Baja California. The Antelope Valley California Poppy Reserve is located in northern Los Angeles County. At the peak of the blooming season, orange flowers seem to cover all 1,745 acres (706 ha) of the reserve. Other prominent locations of California poppy meadows include Bear Valley (Colusa County) and Point Buchon (San Luis Obispo County).

Eschscholzia californica was the first named species of the genus Eschscholzia, named by the German botanist Adelbert von Chamisso after the Baltic German botanist Johann Friedrich von Eschscholtz, his friend and colleague on Otto von Kotzebue's scientific expedition to California and the greater Pacific circa 1810 aboard the Russian ship Rurik.

California poppy is highly variable, with over 90 synonyms. Some botanists accept two subspecies—one with four varieties (e.g., Leger & Rice 2003)—though others do not recognize them as distinct (e.g., Jepson 1993):

Botanical research has held significant implications towards the classification of Eschscholzia. Despite some unresolved aspects in the phylogenies, it is evident that taxonomic are necessary within the genus, particularly in three areas: supporting two subspecies of E. californica, endorsing two subspecies of E. lemmonii, and recognizing two potential new taxa. Currently, 8 taxa are officially recognized.

It belongs to the subfamily Eschscholzioideae, sister group to the subfamily Papaveroideae (which includes Papaver somniferum, the opium poppy).

As poppies are not wind-pollinated, their pollen poses no allergy risk via inhalation.