Sir John Bamford Slack (11 July 1857 – 11 February 1909) was a British politician, member of the Liberal Party and Methodist lay preacher.
He was elected to the House of Commons for the division of St Albans at the St Albans by-election, 1904, replacing Vicary Gibbs.
In 1905, he introduced a bill for women's suffrage, which was talked out.
He received a knighthood.
He married Alice Maud Mary Bretherton (died 1932), who after his death became the first wife of Sir Banister Flight Fletcher.