The first-term Democrat made the announcement today on his Facebook page.
Donnelly joins two Republicans and several Democrats who announced their support for same-sex marriage in recent weeks.
His reversal on the issue comes as the U.S. Supreme Court deliberates whether to overturn federal and state laws that restrict same-sex couples' rights.
In his Facebook post, Donnelly cited his vote to repeal the "Don't Ask, Don't Tell" military policy and his support for legislation that bars workplace discrimination based on sexual orientation.
Here is Sen. Donnelly's full statement:
"In recent years, our country has been involved in an important discussion on the issue of marriage equality. While serving in the House of Representatives, I had the opportunity to act on a core belief of mine: we are a stronger country when we draw on the strengths of all Americans. I voted to repeal 'don't ask, don't tell' and was an original supporter of the bill that would make it illegal to discriminate against someone in the workplace because of their sexual orientation. It is also for that reason that I oppose amending either Indiana's or our nation's constitution to enshrine in those documents an 'us' and a 'them,' instead of a 'we.' With the recent Supreme Court arguments and accompanying public discussion of same-sex marriage, I have been thinking about my past positions and votes. In doing so, I have concluded that the right thing to do is to support marriage equality for all."