Trump lashes out at '13 angry Democrats'
Vermont governor snubs Sanders, backs Clinton
Shumlin tweeted the endorsement on his campaign account on Wednesday afternoon. The tweet ended with a designation that it came directly from the governor, and an aide confirmed its authenticity.
Despite the decision not to back Sanders, the governor's spokesman, Scott Coriell, praised the Vermont senator in a statement to The Hill.
"The Governor has tremendous respect for Senator Sanders and thinks his voice will focus this campaign on the issues that really matter to Vermonters and Americans," he said.
"Hillary Clinton has been fighting her whole life for everyday Americans and the Governor believes she is the right person to lead America as the next president of the United States."
Shumlin isn't the only Vermont politician giving Sanders the cold shoulder.
The state's senior Democratic senator, Patrick Leahy, endorsed Clinton earlier this year, despite having served alongside Sanders in the Senate since 2007.
Clinton has already won endorsements from more than 60 percent of Senate Democrats and 40 percent of House Democrats after her first month of campaigning, according to a tally by The Hill.
Sanders jumped into the race despite long odds and has seized the populist mantle by touting issues such as Wall Street reform and free college. He called Clinton's immense wealth "a problem" during an interview with CNN on Tuesday and previously panned her ties to Wall Street as a concern.
But he's also told CNN on Sunday that he likes Clinton and doesn't want to run a negative campaign against his former Senate colleague.
Clinton holds a commanding lead over Sanders in all Democratic polling, though he has posted strong fundraising numbers since launching his campaign earlier this month.
Sanders pocketed $1.5 million in his first 24 hours, more than any other 2016 presidential candidate who has released fundraising figures, and $3 million in the first four days, according to The Huffington Post.
This story was updated at 5:32 p.m.