The Missouri Democrat's action Sunday came only days before the U.S. Supreme Court is scheduled to consider two major cases on same-sex marriage, Politico reported Monday.
"I have come to the conclusion that our government should not limit the right to marry based on who you love," she wrote on her Tumblr page. "While churches should never be required to conduct marriages outside of their religious beliefs, neither should the government tell people who they have a right to marry."
Sen. Rob Portman, R-Ohio, a former opponent of same-sex marriage, recently announced his support after learning his son was gay.
McCaskill said she opposed gay marriage during a failed run for governor in 2004. Writing Sunday, she said her views had changed, adding "as many of my gay and lesbian friends, colleagues and staff embrace long term committed relationships, I find myself unable to look them in the eye without honestly confronting this uncomfortable inequality."