Marit Arnstad (born 4 May 1962) is a Norwegian lawyer and politician for the Centre Party. She served as the party's parliamentary leader from 2014 to 2025, having previously held the office from 2003 to 2005. Arnstad was the Norwegian Minister of Transport from 2012 to 2013 and Minister of Petroleum and Energy from 1997 to 2000.

Arnstad was born in Stjørdal Municipality, and is the daughter of tax assessor and farmer Arne Arnstad and nurse Aasta Auran, and twin sister of Eli Arnstad. She is also the sister-in-law of Iwar Arnstad, and cousin of Ellen Arnstad. The Arnstad family from Arnstad vestre at Skatval has fostered several Centre Party politicians since the interwar period. She held various jobs and offices before entering politics on a national level in 1993.

She also has a son, August Arnstad (born 1999).

In 1993, she was elected to the Storting and sat as a representative for the Centre Party until 1997. In 2001 she was once again elected to the Storting, where she sat until 2005. She reasoned that she wanted to seek a job outside of politics and focus her attention on raising her son.

She was nominated at the top spot on the Centre Party ballot in Nord-Trøndelag for the 2013 election, which is considered a safe seat. She was preferred for the top spot over Lars Peder Brekk who had the position in 2005. She chaired the Standing Committee on Business and Industry between 2013 and 2014, when she stepped down to become the party's parliamentary leader for a second time. She has also been mentioned as a possible future leader of the Centre Party.

In May 2024, she announced that she wouldn't seek re-election at the 2025 election.

Within the Centre Party, she notably served as its parliamentary leader from 2003 to 2005. She was re-elected to said position again in 2014 when Trygve Slagsvold Vedum became party leader.

Following the 2021 election, she joined party leader Trygve Slagsvold Vedum with pre-government negotiations in Hurdal Municipality that began on 23 September.