John William Casilear (June 25, 1811 – August 17, 1893) was an American landscape artist belonging to the Hudson River School.

By the middle 1830s Durand had become interested in landscape painting through his friendship with Thomas Cole.

By 1840, Casilear's interest in art was sufficiently strong to accompany Durand, John Frederick Kensett, and artist Thomas Prichard Rossiter on a European trip during which they sketched scenes, visited art museums, and fostered their interest in painting.

Casilear gradually developed his talent in landscape art, painting in the style that was later to become known as the Hudson River School.

In 1867 Casilear married Helen M. Howard in the town of Tamworth, New Hampshire.