Christiaan Karel Appel (25 April 1921 – 3 May 2006) was a Dutch painter, sculptor, and poet. He started painting at the age of fourteen and studied at the Rijksakademie in Amsterdam in the 1940s. He was one of the founders of the avant-garde movement Cobra in 1948.

Christiaan Karel Appel was born on 25 April 1921 in his parents' house at Dapperstraat 7 in Amsterdam, Netherlands. On the ground floor, his father, Jan Appel, had a barber shop. His mother, born Johanna Chevalier, was a descendant of French Huguenots. Karel Appel had three brothers.

At fourteen, Appel produced his first real painting on canvas, a still life of a fruit basket. For his fifteenth birthday, his wealthy uncle Karel Chevalier gave him a paint set and an easel. An avid amateur painter himself, Chevalier gave his namesake some lessons in painting.

Appel studied at the Rijksakademie van Beeldende Kunsten in Amsterdam during the German Occupation from 1940 to 1943 and it was there he met the young painter Corneille and, some years later, Constant; they became close friends for years. His parents opposed his choice to become an artist, leading him to leave home; this was also necessary because he needed to hide from the German police so that he would not be picked up and sent to Germany to work in the weapon industry. Appel had his first show in Groningen in 1946. In 1949 he participated with the other CoBrA artists in the Stedelijk Museum Amsterdam; this generated a huge scandal and many objections in the press and public. He was influenced by Pablo Picasso, Henri Matisse and the French brute-art artist Jean Dubuffet. In 1947 he started sculpting with all kinds of used materials (in the technique of assemblage) and painted them in bright colors: white, red, yellow, blue and black. He joined the Experimentele Groep in Holland together with the young Dutch painters Anton Rooskens, Theo Wolvecamp and Jan Nieuwenhuys. Later the Belgian writer Hugo Claus joined the group.