Schweitzer, who has been critical of front-runner Hillary Clinton for many of the same reasons as O'Malley, will also serve as a national co-chair of the former Maryland governor's campaign.
"Martin and I worked closely together as Democratic Governors, and I saw firsthand how he didn’t just talk about his progressive values -- he actually put them into action," Schweitzer said in a statement. "Martin and I don't agree on every issue, but he is the only candidate with a proven record of getting things done."
Schweitzer's endorsement comes at a critical moment for O'Malley's long shot bid. Clinton's polling has improved since last week's Democratic debate while O'Malley remains in single digits.
With former Virginia Sen. Jim Webb and former Rhode Island Gov. Lincoln Chafee dropping out, O'Malley is trying to push back on the narrative that the contest is increasingly a two-person race between Clinton and Vermont Sen. Bernie Sanders.
O'Malley will have an opportunity to make that case Saturday when he speaks to the Iowa Jefferson Jackson dinner, the state’s largest gathering of Democrats.