John Hassall RI was an English illustrator, known for his advertisements and poster designs.

Hassall was born in Walmer, Kent, and was educated in Worthing, at Newton Abbot College, and at Neuenheim College, Heidelberg. After twice failing entry to The Royal Military Academy Sandhurst, he emigrated to Manitoba in Canada in 1888 to begin farming with his brother Owen. He returned to London two years later when he had drawings accepted by the Graphic. At the suggestion of Dudley Hardy (along with Cecil Aldin, a lifelong friend), he studied art in Antwerp and Paris. During this time he was influenced by the famous poster artist Alphonse Mucha.

In 1895, he began work as an advertising artist for David Allen & Sons, a career which lasted fifty years and included such well-known projects as the poster "Skegness Is so Bracing" (1908). Between 1896 and 1899 alone, he produced over 600 theatre poster designs for this firm while, at the same time, providing illustrations to several illustrated newspapers. Making use of flat colours enclosed by thick black lines, his poster style was very suitable for children's books, and he produced many volumes of nursery rhymes and fairy stories, such as Mother Goose’s Nursery Rhymes (1909).

In 1901, Hassall was elected to the membership of the Royal Institute of Painters in Water Colours and the Royal Society of Miniature Painters. He also belonged to several clubs, including the Langham (until 1898), the Savage, and the London Sketch Club, of which he was a President from 1903-1904. He belonged to the literary club The Sette of Odd Volumes and illustrated their privately printed menus, including one of a broken bust of Jane Austen for the club's "Night of the Divine Jane" in 1902.

In 1900, Hassall opened his own New Art School and School of Poster Design in Kensington where he numbered Annie Fish, Bert Thomas, Bruce Bairnsfather, H. M. Bateman and Harry Rountree among his students. The school was closed at the outbreak of the First World War. In the post-war period, he ran the very successful John Hassall Correspondence School.

John Hassall was the father of poet Christopher Hassall and the printmaker Joan Hassall, OBE. He was also the grandfather of the actress Imogen Hassall.