Mona Ingeborg Sahlin (Swedish pronunciation: [ˈmôːna saˈliːn]; née Andersson; born 9 March 1957) is a Swedish politician who was leader of the opposition and leader of the Swedish Social Democratic Party from 2007 to 2011.

Sahlin was a Member of Parliament, representing Stockholm County, from 1982 to 1996 and again from 2002 to 2011. She has also held ministerial posts in the Swedish government from 1990 to 1991, from 1994 to 1995 and from 1998 to 2006. Sahlin was elected as leader of the Social Democratic Party on 17 March 2007, succeeding Göran Persson who resigned as leader following the defeat in the 2006 general election. Sahlin is the first female leader of the Swedish Social Democratic Party and became in 2011 the first since Claes Tholin in 1907 to leave that position without having served as Prime Minister of Sweden. In 2012, her successor Håkan Juholt joined her as the second now living person to do so. On 14 November 2010, following another electoral defeat for the Social Democrats, she announced her intent to step down as party chairman, which she did in early 2011.

Sahlin was born Mona Ingeborg Andersson in Sollefteå, Västernorrland County, Sweden. Her father, Hans Andersson, worked at youth care schools community homes. In the mid-1960s, the family moved to Järla in Stockholm County where they remained. Her father later became an advisor to former Prime Minister Ingvar Carlsson.

In 1964, at the age of seven, Sahlin founded the Swedish "Barbie Club". During her childhood, she also enjoyed soccer and music. Sahlin performed as one of the back up singers to Jan Malmsjö, in the selection for the song to represent Sweden in the Eurovision Song Contest 1969. The song was written by Benny Andersson and Lasse Berghagen and it came in second place.

Sahlin was educated at Nacka Samskola and Södra Latin in Stockholm and completed secondary school in 1977. From 1976 to 1977, she was vice chairperson of the Swedish Pupils' Association. Thereafter she worked at a private company and later as a trade union representative for the Swedish National Union of State Employees.

At age 13, Sahlin joined the Swedish support group for the Viet Cong. Sahlin's political career began in the Swedish Social Democratic Youth League in Nacka, Stockholm County, in 1973, at the age of 16. This was during the Vietnam War, and already as a 13-year-old Sahlin had joined the Swedish FNL group.

In the Swedish general election of 1982, Sahlin was elected to the Riksdag as the youngest member of parliament at that time. In 1990, she became Minister for Employment, but after the Social Democrats lost power in the 1991 election, Sahlin began to serve as chairman of the Riksdag's Committee on the Labour Market and as spokesman for the Social Democrats on labour market issues. From 1992 to 1994, she was party secretary. During this period, she openly criticized government reforms, particularly on social welfare and employees' rights, maintaining they needed to be reversed. She left her position to rejoin the government as Minister for Gender Equality and Deputy Prime Minister, when the Social Democrats regained power in the 1994 election.

In October 1995, the newspaper Expressen following an investigation led by Christian Democratic Spanish-Swedish Public Auditor Carlos Medina de Rebolledo reported that Sahlin, who was then serving as Deputy Prime Minister and was widely seen as the main candidate to succeed Ingvar Carlsson as Prime Minister, had charged SEK 53,174 for private expenses on her working charge card, which was only for working expenses. At a news conference, she admitted that she had used a Government credit card to buy groceries. She further confessed to having failed to pay 19 parking tickets and several bills for her children's day care on time. Later, she apologized in a Stockholm newspaper. A preliminary investigation was initiated by the chief prosecutor Jan Danielsson [sv], as a result of the transactions, and was closed in early 1996 when it came to the conclusion that there was no infringement. She eventually paid the bills (along with an extra of SEK 15,000, which was later returned) to the Treasury. The controversy was dubbed as the "Toblerone affair" due to the inclusion of Toblerone bars on the credit card statement.