Mark Brandt Dayton (born January 26, 1947) is an American politician who served as the 40th governor of Minnesota from 2011 to 2019. He served as a United States Senator representing Minnesota from 2001 to 2007 and as Minnesota State Auditor from 1991 to 1995. He is a member of the Minnesota Democratic–Farmer–Labor Party (DFL), which affiliates with the national Democratic Party.

Dayton is the great-grandson of businessman George Dayton, the founder of Dayton's, a department store that later became the Target Corporation. He embarked on a career in teaching and social work in New York City and Boston after graduating from Yale University in 1969. During the 1970s, he served as a legislative aide to U.S. Senator Walter Mondale and Minnesota Governor Rudy Perpich. In 1978, Dayton was appointed the Minnesota Economic Development Commissioner and married Alida Rockefeller Messinger, a member of the Rockefeller family. Dayton ran for the U.S. Senate in 1982 against Republican Party incumbent David Durenberger. He campaigned as a populist in opposition to Reaganomics and famously promised "to close tax loopholes for the rich and the corporations—and if you think that includes the Daytons, you're right." Durenberger won the election, and Dayton returned to the Perpich administration until his election as Minnesota State Auditor in 1990.

In 1998, Dayton ran for governor, losing the Democratic nomination to Hubert Humphrey III. In 2000, he was elected to the U.S. Senate, defeating Republican incumbent Rod Grams. As senator, Dayton voted against the authorization for the Iraq War and introduced legislation creating a cabinet-level United States Department of Peace. In 2006, he chose not to seek reelection, citing his disillusionment with Washington, D.C., and fundraising.

In 2010, Dayton defeated Republican Tom Emmer to become governor of Minnesota despite national success for the Republican Party, including in the Minnesota legislature. He won a second term in 2014 over Republican opponent Jeff Johnson and opted not to run for a third term in 2018. His major legislative initiatives during his governorship include the legalization of same-sex marriage and the construction of U.S. Bank Stadium.

Dayton was born on January 26, 1947, in Minneapolis and is the eldest of Gwendolen May (Brandt) and Bruce Bliss Dayton's four children. He is a great-grandson of businessman George Dayton, the founder of the Dayton's department store chain. His father, Bruce Dayton, served as the chairman and CEO of Dayton Hudson Corporation, the company that later became the Target Corporation. Bruce Dayton also founded the B. Dalton bookstore chain in 1966.

Mark Dayton was raised in Long Lake, Minnesota, and graduated from the Blake School in Minneapolis, where he was an all-state ice-hockey goaltender as a senior.

Dayton attended Yale University, where he played varsity hockey until an accident on the ice. During his time at Yale, he joined the Delta Kappa Epsilon fraternity (alongside future President George W. Bush) and received his Bachelor of Arts in psychology in 1969. After college, Dayton worked as teacher on the Lower East Side of Manhattan in New York City from 1969 to 1971, and then as the chief financial officer of a social service agency in Boston from 1971 to 1975. He married his first wife, Alida Rockefeller, in 1978.

Dayton first became politically active in the 1960s. He protested the Vietnam War in April 1970 at one of Minnesota's major antiwar protests against Honeywell, where he was maced by police. Dayton's father served on the Honeywell board of directors and the two had a strained relationship after the incident.