Stanislao Cannizzaro (July 13, 1826 - May 10, 1910) was an Italian chemist.

Cannizzaro was born in Palermo. In 1841 he entered the university of his native place with the intention of making medicine his profession, but he soon turned to the study of chemistry, and in 1845 and 1846 acted as assistant to Raffaele Piria (1815-1865), known for his work on salicin, who was then professor of chemistry at Pisa and subsequently occupied the same position at Turin. During the Sicilian revolution he served as an artillery officer at Messina and was also chosen deputy for Francavilla in the Sicilian parliament; and after the fall of Messina in September 1848 he was stationed at Taormina. On the collapse of the insurgents he escaped to Marseilles, in May 1849, and after visiting various French towns reached Paris in October. There he gained an introduction to Michel-Eugene Chevreul's laboratory, and in conjunction with F. S. Cloz (1817-1883) made his first contribution to chemical research in 1851, when they prepared cyanamide by the action of ammonia on cyanogen chloride in ethereal solution. In the same year he was appointed professor of physical chemistry at the National College of Alessandria, Piedmont, where he discovered that aromatic aldehydes are decomposed by an alcoholic solution of potassium hydroxide into a mixture of the corresponding acid and alcohol, e.g. benzaldehyde into benzoic acid and benzyl alcohol, the Cannizzaro reaction. In the autumn of 1855 he became professor of chemistry at Genoa university, and after further professorships at Pisa and Naples, accepted the chair of inorganic and organic chemistry at Palermo. There he spent ten years, studying the aromatic compounds and continuing to work on the amines, until in 1871 he was appointed to the chair of chemistry at Rome university.

Apart from his work on organic chemistry, which includes also an investigation of santonin, he rendered great service to the philosophy of chemistry when in his memoir Sunto di un corso di Filosofia chimica (1858) he insisted on the distinction, previously hypothesised by Avogadro, between molecular and atomic weights, and showed how the atomic weights of elements contained in volatile compounds can be deduced from the molecular weights of those compounds, and how the atomic weights of elements of whose compounds the vapour densities are unknown can be ascertained from a knowledge of their specific heats. For this achievement, of fundamental importance for the atomic theory in chemistry, he was awarded the Copley Medal by the Royal Society in 1891. Cannizzaro's scientific eminence in 1871 secured him admission to the Italian senate, of which he was vice-president, and as a member of the Council of Public Instruction and in other ways he rendered important services to the cause of scientific education in Italy.