James Sidney Ensor, Baron Ensor (April 13, 1860–November 19, 1949), was a Belgian painter whose unique portrayals of grotesque humanity made him a principal precursor of 20th-century expressionism and surrealism.

Self-portrait on a Belgian poststamp

Ensor was born in Ostend, Belgium, in 1860, and — except for three years spent at the Brussels Academy, from 1877 to 1880 — he lived in Ostend all his life. His early works were of traditional subjects: landscapes, still lifes, portraits, and interiors painted in deep, rich colors and enriched by a subdued but vibrant light. In the mid-1880s, influenced by the bright color of the Impressionists and the grotesque imagery of earlier Flemish masters such as Hieronymus Bosch and Pieter Bruegel the Elder, Ensor turned toward avant-garde themes and styles. He took his subject matter principally from Ostend's holiday crowds, which filled him with revulsion and disgust. Portraying individuals as clowns or skeletons or replacing their faces with carnival masks, he represented humanity as stupid, smirking, vain, and loathsome. Outstanding in this vein is his immense canvas Christ's Entry Into Brussels in 1889 (1888, J. Paul Getty Center, Los Angeles, California).

Ensor deliberately used harsh, garish colors and violent, broken brushstrokes to heighten the violent effect of his subjects. His work had an important influence on 20th-century painting, his lurid subject matter paving the way for surrealism and Dada, and his techniques — particularly his brushwork and his coloristic sense — leading directly to expressionism. He died in 1949 in Ostend, where there is now a museum devoted to his work.

In 1994 a new audience was introduced to James Ensor when They Might Be Giants released the song "Meet James Ensor," which aptly describes him as "Belgium's famous painter."

In 1995, the state of Belgium recognized Ensor's achievements by dedicating the 100-franc (~ 2.5 EUR) bill to him and his work.