Thomas Alexander Harrison, was an American marine painter who spent most of his career in France.

He studied at the Pennsylvania Academy of Fine Arts in Philadelphia, 1871-72. For nearly six year he worked as a draftsman for a United States government survey expedition mapping the Pacific coast. He studied for a short time at the San Francisco School of Design. In 1879, he moved to Paris and studied at the École nationale supérieure des Beaux-Arts under Jean-Léon Gérôme and Jules Bastien-Lepage.

Chafing under the restraints of the schools, he traveled to Brittany, where at Pont-Aven and Concarneau he turned his attention to marine painting and landscape.

A figure-piece he sent to the 1882 Salon attracted attention, a boy daydreaming on the beach, which he called Châteaux en Espagne (Castles in Spain) (1882, Metropolitan Museum of Art). In the 1885 Salon, he had a large canvas of several nude women called En Arcadie (1885, Musée d'Orsay), a remarkable study of flesh tones in light and shade which had a strong influence on the younger men of the day. This received an honourable mention, the first of many awards conferred upon him. Les Amateurs (1882–83, Brauer Museum of Art), was awarded a first medal at the 1889 Paris Exhibition. Other honors included the 1887 Temple Gold Medal of the Pennsylvania Academy of Fine Arts, and medals in Munich, Brussels, Ghent, Vienna and elsewhere.

He was decorated by the Legion of Honour, and was an officier of Public Instruction, Paris. He was a member of the Société Nationale des Beaux-Arts, Paris; of the Royal Institute of Painters in Oil Colours, London; of the Secession societies of Munich, Vienna and Berlin; of the National Academy of Design, the Society of American Artists, New York, and other art bodies.

His reputation rests on marine pictures such as The Wave (1885, Pennsylvania Academy of the Fine Arts), with long waves rolling in on the beach, and great stretches of open sea under poetic conditions of light and colour.

His brother, L. Birge Harrison (1854–1929), was also a painter. Another brother, Butler Harrison (died 1886), was a figure painter.