The endorsement has not been finalized yet, as Sanders pushes for expanded health care access. The Vermont senator is expected to make the announcement at a campaign event in New Hampshire on Tuesday.
In a speech to supporters last month, Sanders vowed to help Clinton defeat Republican Donald Trump in the Nov. 8 election but did not end his campaign.
Sanders has resisted endorsing Clinton in a show of party unity since she clinched the Democratic nomination last month. Sanders chose to continue his campaign as leverage to win concessions on his liberal policy agenda and reforms to the Democratic Party nominating process.
On Wednesday, Clinton unveiled a higher education plan that would eliminate tuition for families with incomes up to $125,000 at in-state public colleges and universities.
The policy is also a win for Sanders, who campaigned on a platform to make tuition free at public colleges and universities. The Vermont senator praised Clinton's plan on Twitter, saying it was a "very bold initiative."
@BernieSanders: I applaud @HillaryClinton for the very bold initiative she has just brought forth for the financing of higher education.
— Reuters and CNBC's John Harwood and Ivan Levingston contributed to this report.