The move by the Senate Conservatives Fund came two days after the incumbent, Senate Republican Leader Mitch McConnell, finalized a budget compromise with Democrats to end an impasse that had led to a partial shutdown of the federal government.
The group, founded by former Sen. Jim DeMint, was sharply critical of that budget deal.
In announcing the endorsement, Executive Director Matt Hoskins called Bevin "a true conservative" who will fight to stop overspending, bailouts and rising debt.
"We know that winning this primary won't be easy," Hoskins said. "Mitch McConnell has the support of the entire Washington establishment, and he will do anything to hold on to power. But if people in Kentucky and all across the country rise up and demand something better, we're confident Matt Bevin can win this race."
Bevin is far behind in fundraising and would benefit from the group's national network to bolster his campaign bank account.
McConnell has already collected $17.7 million for his re-election bid, and still had $10 million cash on hand. Bevin has raised $222,000 and kicked in a $600,000 personal loan.
The Senate Conservatives Fund already has spent some $340,000 to run TV ads in Kentucky criticizing McConnell for not doing enough in the push to defund President Barack Obama's health care reforms.
McConnell spokeswoman Allison Moore discounted the group's endorsement.
"Matt Bevin now has the dubious honor of standing with a self-serving DC fundraising group that made its name by recruiting and promoting unelectable candidates that ensured Barack Obama a majority in the Senate," Moore said. "They clearly care less about Kentuckians than they do about their reputation for supporting laughably bad candidates."
Bevin said he's humbled to have the endorsement from an organization that's fighting for the same conservative values he is.
"Mitch McConnell has not lived up to those values during his nearly 30 years in Washington, and it is time for a change," Bevin said.