"Montanans believe in the right to make a good life for their families. How they define a family should be their business and their business alone," Tester wrote. "I’m proud to support marriage equality because no one should be able to tell a Montanan or any American who they can love and who they can marry."
Tester's statement follows similar announcements from a number of senators in recent days. Democratic Sens. Mark Begich (Alaska), Claire McCaskill (Mo.), Jay Rockefeller (W.Va.), and Mark Warner (Va.) have all come out in support of same-sex marriage.
Those announcements follow two other similar statements by Sen. Rob Portman (R-Ohio) and former Secretary of State Hillary Clinton who, about a week earlier, both said they support same-sex marriage.
Tester's announcement on Tuesday came the same day that the Supreme Court began hearing arguments on a case on same-sex marriage.
Sen. Tester: I'm 'proud' to support same-sex marriage
Tester announced his support in a message on his Facebook on Tuesday.