Thorald Læssøe was a Danish landscape painter, son of Margrethe Juliane Signe Læssøe, brother of the officer Frederik Læssøe, the museum man Ludvig Læssøe and the priest Kristian Frederik Læssøe.

Thorald Læssøe was first destined to become a farmer. When he wanted more to be an artist, he became a student of animal painter Christian Frederik Carl Holm until his departure for Munich. He visited the Academy of Fine Arts, but trained mostly on his own as a landscape and architecture painter. He was friends JA Jerichau, J.Th. Lundbye and Lorenz Frølich, but did not reach any of his famous friends in artistic skill or reputation; but he was a gracious man and in many respects a gifted nature with a fine sense of art. Jerichau supported him with all his might, to receive the Academy of Fine Arts' smaller travel scholarship for one year. After returning home, he married in 1857 Emy Francisca Erhardine Tidonia Countess Krag-Juel-Vind-Frijs (1825-1863), daughter of Chamberlain Jens Christian Carl Count Krag-Juel-Vind-Frijs to Frijsenborg . After his trip abroad, he exhibited landscapes and architectural images from the southern European regions.