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Jean Baptiste Julien d'Omalius d'Halloy (17 February 1783 in Liège – 15 January 1875 in Brussels) was a Belgian statesman and geologist. He was the first to define the Cretaceous as a distinct geological period, in 1822. He produced the first geological map of France, the Benelux, the Rhineland and Switzerland, completed in 1813 and published in 1822. Halloysite, a clay mineral, was named in his honour. He also wrote on races.
He was a member of the Royal Academy of Belgium (elected on July 3, 1816 and president in 1850, 1858 and 1872), president of the Geological Society of France (1852) and corresponding member of the French Academy of Sciences (1842). He was made a foreign member of the Royal Society in 1873.
Halloy was governor of the province of Namur during the period of the United Kingdom of the Netherlands (1815-1830). He was elected to the Belgian Senate in 1848, of which he became vice-president three years later (1851), a position he held until 1870 making him the longest serving vice-presidents of the Senate in Belgian history.
He had two daughters. His daughter Sophie married on February 27, 1838 Baron Edmond de Selys Longchamps, vice-president of the Senate of Belgium, renowned entomologist, president of the Royal Society of Sciences of Liège.