Robert Archibald Armstrong, LL.D. (1788–1867), was a Gaelic lexicographer. He was the eldest son of Robert Armstrong, of Kenmore, Perthshire, by his wife, Mary McKercher. He was born at Kenmore in 1788, and educated partly by his father, and afterwards at Edinburgh and at St. Andrews University, here he graduated. Coming to London from St. Andrews with high commendations for his Greek and Latin acquirements, he engaged in tuition, and kept several high-class schools in succession in different parts of the metropolis. He devoted his leisure to the cultivation of literature and science. Of his humorous articles "The Three Florists," in Eraser for January 1838, and "The Dream of Tom Finiarty, the Cab-driver," in the Athenæum, are notable examples. His scientific papers appeared chiefly in the Arcana of Science and Art (1837 et seq.), and relate to meteorological matters. But his great work was A Gaelic Dictionary, in two parts — I. Gaelic and English, II. English and Gaelic — in which the words, in their different acceptations, are illustrated by quotations from the best Gaelic writers, London, 1825, 4to, This was the first Gaelic dictionary published, as there previously existed only vocabularies of the language like those of William Shaw (An Analysis of the Galic Language 1778 and Galick and English Dictionary 1780) and others.