The terms California Impressionism and California Plein-Air Painting describe the large art movement of 20th century artists who worked out of doors (en plein air), directly from nature in California, United States. Their work became popular in the San Francisco Bay Area and Southern California in the first three decades after the turn of the 20th century. Considered to be a regional variation on American Impressionism, the California Impressionists are a subset of the California Plein-Air School.

The California Impressionist artists depicted the California landscape from the south to the north—the foothills, mountains, seashores, and deserts of the interior and coastal regions. California Impressionism reached its peak of popularity in the years before the Great Depression. The California Impressionists generally painted in a bright, chromatic palette with loose, painterly brushwork that showed influence from French Impressionism and Post-Impressionism. These artists gathered in art colonies in places like Carmel-by-the-Sea and Laguna Beach as well as in cities like San Francisco, Los Angeles, Pasadena, and even Giverny. The artists associated with California Impressionism were influenced by European Impressionism, and members such as Guy Rose and Alson Clark were educated and closely associated with European Impressionists.

Organizations like the California Art Club, the Painters and Sculptors Club, San Francisco's Sketch Club, The Carmel Art Association, The Laguna Beach Art Association and the Los Angeles Museum of History, Art and Science played a key role in popularizing the work of the California Impressionists. While impressionist-influenced painting remained popular in California well after it did in Europe or the Eastern United States, in the late 1920s, impressionism was seen as old-fashioned and conservative. In the 1930s, more modern styles became accepted, and the movement fell into decline.

In the 1890s, European ideas and painting techniques finally made their way to the west coast of the United States. We call the group of artists who responded to these ideas the California Impressionists, but that is not a name that they self-applied. Charles Desmarais, director of the Laguna Art Museum, noted that this phrase "is a misnomer if it implies any but the most tenuous connections to the theory-based art born in France decades before".

Patricia Trenton argued that the "common thread that binds these artists is the recognition and depiction of light through color, an interest fostered in part by the dissemination of the color theories of Michel-Eugene Chevreul and Albert Henry Munsell. But... these artists were inspired by the clarity and force of the distinctive light of Southern California and by the region's endlessly intriguing motives of hill and meadow, desert and mountain, river and ocean".

Most of the Plein Air painters came from the East, the Midwest and Europe, and only a few of the early artists such as Guy Rose were actually born and raised in California. Most of these artists were already trained in art when they moved to California, arriving between 1900 and the early 1920s, although many continued their education in California. These artists worked together and showed together frequently.

Many of the Northern California painters were influenced by the works of the French painters of the Barbizon School, who worked in the forest south of Paris in the mid-19th century, as well as the American landscape master George Inness (1825–1894) and the American expatriate James Abbott McNeill Whistler (1834–1903). Northern California Tonalist landscapes can be recognized by their simplified compositions and a limited palette that gave the paintings close color harmonies. Some of the other major Northern California Tonalists were Arthur and Lucia Mathews, who led the Bay Area Arts and Crafts Movement, the moonlight painter Charles Rollo Peters (1862–1928), the flamboyant Xavier Martinez (1869–1943), and the painter and muralist Giuseppe Cadenasso (1858–1918). While many of the Northern California tonalist painters did paint extensively out of doors, most of the works were done in their studio, stylized and poetic visions, a step away from the plein air or impressionist painting favored by the French school.

After 1915 and the Panama–Pacific International Exposition, which brought many French and American Impressionist masterworks to San Francisco, more Northern California painters adopted a more chromatic palette and dappled brushwork that was closer to French Impressionism and they adopted high-key midday subjects. Some of the best known Northern California painters who worked in a more impressionistic manner were the marine painter Armin Hansen, the coastal landscape painter Bruce Nelson and E. Charlton Fortune (1885–1969), a talented Monterey woman who gave up easel painting for ecclesiastical decoration. Joseph Raphael, a student of Arthur Mathews who lived for many years in Europe while maintaining ties to San Francisco, assayed methods of Impressionism and Post-Impressionism, and may have been "the finest and most original of the state's Impressionists."