Oskar Lafontaine (German pronunciation: [ˈlafɔntɛn]; born 16 September 1943) is a German politician. He served as Minister-President of the state of Saarland from 1985 to 1998, and was federal leader of the Social Democratic Party (SPD) from 1995 to 1999. He was the lead candidate for the SPD in the 1990 German federal election, but lost by a wide margin. He served as Minister of Finance under Chancellor Gerhard Schröder after the SPD's victory in the 1998 federal election, but resigned from both the ministry and Bundestag less than six months later, positioning himself as a popular opponent of Schröder's policies in the tabloid press.

In the lead-up to the 2005 federal election, as a reaction to Schröder's Agenda 2010 reforms, Lafontaine co-founded the left-wing party Labour and Social Justice – The Electoral Alternative. Following a merger with the Party of Democratic Socialism in June 2007, he became co-chairman of The Left. He was the lead candidate for the Saarland branch of the party in the 2009 Saarland state election where it won over 20% of the vote. He announced his resignation from all federal political functions after being diagnosed with prostate cancer in 2009. He retained his position as a member of the Saarland legislature, and since May 2012 has been leader of the opposition in Saarland. Lafontaine resigned from the Left Party on 17 March 2022 because it was no longer an "alternative to the politics of social insecurity and inequality," he said.