Alexander John Forbes-Leith, 1st Baron Leith of Fyvie JP, DL (6 August 1847 – 14 November 1925), was a Scottish Royal Navy officer and US steel magnate.

[1] Alexander was the youngest of four children born to Rear-Admiral John James Leith and his wife and step-niece, Margaret Forbes.

He was educated at Berlin, Prussia, the École spéciale militaire de Saint-Cyr and Dr. Burney's Naval Academy at Gosport, Hampshire.

In 1889, Forbes-Leith used the fortune he had made in the steel industry to acquire Fyvie Castle in Aberdeenshire for £175,000 and invested large sums in its restoration.

Lord Leith's estates, including Fyvie Castle, passed to his daughter and only surviving child, the Honourable Ethel, wife of the former Conservative Member of Parliament Sir Charles Rosdew Burn, 1st Baronet.

In 1925, the latter assumed the surname and arms of Forbes-Leith of Fyvie, for himself, his wife and son, according to the terms of his father-in-law's will (see Forbes baronets for more information on the baronetcy).