U.S. District Judge Barbara Crabb issued the ruling Friday, but it wasn't clear whether same-sex marriages could immediately begin.
But the ruling makes Wisconsin the 27th state where same-sex couples can marry under law or where a judge has ruled they ought to be allowed to wed.
Clerks in the state's two largest cities of Milwaukee and Madison had been preparing for such a ruling by bringing in extra staff to handle an expected flood of marriage-license applicants.
The lawsuit was brought by the American Civil Liberties Union on behalf of eight same-sex couples.
Gov. Scott Walker has been a strong proponent of the constitutional same-sex marriage ban that was approved by state voters in 2006.
Read the entire decision, below:
Copyright 2014 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.