Jean Baptiste Julien d'Omalius d'Halloy (February 17, 1783 in Liège – January 15, 1875 in Brussels) was a Belgian geologist. He also wrote on races.
Born in Liège, he was the only son of an ancient and noble family, and his education was carefully directed. After completing his classical studies in his home town he was sent to Paris in 1801 by his parents to avail himself of the social and literary advantages of the metropolis. A lively interest, however, in geology awakened by the works of Buffon, directed his steps to the museums and the Jardin des Plantes.
He visited Paris again in 1803 and 1805, and during these periods attended the lectures of Fourcroy, Lacépède, and Georges Cuvier. His homeward journeys were usually made the occasion of a geological expedition through northern France. As early as 1808 he communicated to the Journal des Mines a paper entitled Essai sur la géologie du Nord de la France. He thus conceived the project of making a series of surveys throughout the whole country. This was furthered by a commission to execute a geological map of the empire which brought with it exemption from military duty.
He devoted himself energetically to the work and by 1813 had traversed over 15,500 miles in France and portions of Italy. His family had, however, but little sympathy with his geological activity, and persuaded him to give up his expeditions. The map which he had made of France and the neighbouring territories was not published until 1822 and served as a basis for the more detailed surveys of Armand Dufrénoy and Elie de Beaumont. After having served as sous-intendant of the arrondissement of Dinant (1814) and general secretary of the province of Liège (1815), he became in 1815 governor of Namur. He held this office until after the Revolution of 1830. He was elected a member of the Belgian Senate in 1848, became its vice-president in 1851, was made a member of the Academy of Brussels in 1816, and was elected its president in 1850.