John Brownlee Lonsdale, 1st Baron Armaghdale (23 March 1850 – 8 June 1924), known as Sir John Lonsdale, Bt, between 1911 and 1918, was a British businessman and Conservative politician.
Born at The Pavilion, in the cathedral city of Armagh, Lonsdale was the son of James Lonsdale DL, JP (1826–1913) and Jane Brownlee, and was educated privately. He was a partner in the family firm of J. and J. Lonsdale & Company.
Lonsdale was a director of the Lancashire and Yorkshire Bank and of the North of England Debenture Company, chairman of Levenstein Ltd and vice-chairman of the Manchester Ship Canal Warehousing Company. In 1895 he was appointed High Sheriff of Armagh, succeeding William Maynard Sinton. He was elected a Member of Parliament for Mid-Armagh in a by-election in February 1900, and sat until 1918. During his time in parliament, he acted as a whip for the Irish Unionist Party in the House of Commons for 15 years, and was a strong opponent of Home Rule. He was created a baronet, of Prince's Gardens in the Royal Borough of Kensington and of The Pavilion in the City and County of Armagh, on 7 July 1911, and raised to the peerage as Baron Armaghdale, of Armagh in the County of Armagh, on 17 January 1918. Between 1920 and 1924 he was Lord Lieutenant of Armagh.
Lord Armaghdale married Florence Rumney, of Stubbins House, Lancashire. They had no children. The Armaghdales lived in England at The Dunes, Sandwich Bay, and kept a London house at Prince's Gardens, SW7. A keen golfer, Lord Armaghdale presented the Lonsdale Cup to the County Armagh Golf Club. He died in January 1924, aged 74, when the baronetcy and barony became extinct. Lady Armaghdale died in London in 1937.