They are the latest of several mainline Protestant denominations to take a position against the controversial ballot measure.
Minnesota's United Methodists went on record Friday against changing the state Constitution to define marriage as a union between a man and a woman.
At the group's annual assembly in St. Cloud, 569 members voted on a resolution opposing the amendment. The resolution prevailed in a written ballot, 400 to 169, according to a statement from the Minnesota Conference of the United Methodist Church.
The amendment -- which would underscore the state's statutory ban on gay marriage -- will go before voters Nov. 6.
A group of congregations submitted the resolution, drawing on the United Methodist Book of Discipline, which states "all persons, regardless of age, gender, marital status, or sexual orientation, are entitled to have their human and civil rights ensured and to be protected against violence."
"They argue that many civil rights are based on one's marital status: health insurance, equal taxation, retirement benefits, and health-care directives," the statement said.
The resolution is advisory and does not obligate individual Methodists or their churches, church leaders said.
Minnesota has 72,000 members of 353 United Methodist churches.
The Methodists follow several other Minnesota religious groups that have weighed in on the controversial measure.
Last month, about 430 Lutherans representing 115 congregations in the St. Paul Area Synod of the Evangelical Lutheran Church in America voted on a resolution opposing the marriage amendment at the group's annual assembly at a Burnsville church.
Four other ELCA synods in Minnesota have also adopted similar resolutions in opposition to the amendment.
On the other side of the debate, Catholic bishops representing Minnesota's largest single denomination, with close to 1.1 million members, support the amendment. An increasing number of conservative evangelical Protestant churches and leaders have also spoken out in favor of the amendment.