Alexander Cavalié Mercer (28 March 1783 – 9 November 1868) was a British artillery officer. Although he rose to the rank of general, his fame is as commander of G Troop Royal Horse Artillery in the thick of the fighting at the Battle of Waterloo, and as author of Journal of the Waterloo Campaign. Mercer's Journal is an important source for historians of the Waterloo campaign, as well as a detailed description of the landscape and people of Belgium and France in the early 19th century. It is one of the few accounts of the period written by an artillery officer.