Marit Nybakk (born 14 February 1947, in Nord-Odal) is a Norwegian politician for the Labour Party, a former First Vice-president of the Norwegian Parliament, the Storting, and a former President of the Nordic Council. From 2016 to 2018 she was President of the Norwegian Association for Women's Rights, the preeminent women's and girls' rights organisation in Norway.

A pragmatic social democrat and a proponent of the Third Way, she became a Member of Parliament in 1986 as the substitute for Gro Harlem Brundtland when the latter became prime minister. In 2009 she became the Storting's Third Vice President, before becoming First Vice President in 2013. She did not stand for reelection in 2017; at that point she was both Norway's longest-serving incumbent member of parliament and the longest-serving woman of all times. Nybakk served as President of the Nordic Council for the term 2013.

Nybakk has been one of the Labour Party's principal politicians in foreign and defence affairs since the 1990s and has been her party's spokesperson on defence. She was Chairman of the Standing Committee on Defence between 2001 and 2005 and Vice Chairman of the Standing Committee on Foreign Affairs between 2005 and 2009. She is a former leader of the Socialist Group in the NATO Parliamentary Assembly.

Nybakk studied English, French, and political science in Oslo, Paris and Cambridge, and graduated with the cand.mag. degree at the University of Oslo in 1972. She also studied theatre and drama in Stratford-upon-Avon in 1974. As a student, she was a member of the university board of the University of Oslo, as the first woman to serve in that body. She was President of the Norwegian Students' and Academics' International Assistance Fund 1977–1981.

Nybakk was elected as a deputy member of the Parliament of Norway, the Storting, for the Oslo constituency in 1985. As the deputy of Gro Harlem Brundtland, she became a substitute member of parliament in 1986 when Brundtland became prime minister. She became a full representative in 1987, following the death of Foreign Minister Knut Frydenlund. Nybakk was reelected as a member of parliament for the Oslo constituency in 1989, 1993, 1997, 2001, 2005, 2009, and 2013. She has for several years been the Labour Party's second candidate from the Oslo constituency, second only to party leader and Prime Minister (now NATO secretary-general) Jens Stoltenberg.

Since the 1990s, Nybakk has focused on foreign and defence policy, and has been one of the Labour Party's principal politicians covering this field. She was Chairman of the Standing Committee on Defence between 2001 and 2005 and Vice Chairman of the Standing Committee on Foreign Affairs between 2005 and 2009. She is a member of the Standing Committee on Foreign Affairs and Defence and the European Committee, and Chairman of the Preparatory Credentials Committee during the term 2013–2017. Nybakk has also been the Labour Party's spokesperson on defence.

Nybakk was Third Vice President of the Storting during the term 2009–2013, and was elected First Vice President in 2013.

In 2013 she became both the longest-serving incumbent member of the Norwegian parliament and the longest-serving woman of all times.