Albert Sidney Hornby, usually just A. S. Hornby (1898-1978) was an English grammarian, lexicographer and EFL teacher.

He was born in Chester, and educated at University College London. From 1923 he taught English in Tokyo. In 1937, with E. V. Gatenby and H. Wakefield he began work on a new type of dictionary that was aimed at foreign learners of English, the first monolingual learners' dictionary. It was completed in 1940 and published in 1942 in Tokyo as The Idiomatic and Syntactic English Dictionary. After Hornby had left Japan in 1939, he joined the British Council and after the war he became the editor of English Language Teaching. In 1948 his dictionary was reissued by Oxford University Press as A Learner's Dictionary of Current English. The subsequent, revised editions of the dictionary were probably the greatest commercial success in ELT publishing.