The crowd in Las Vegas broke into applause as Clinton reached over to shake his hand and told him: "Thank you, Bernie."
It was a big night for Clinton, who gave a strong performance in the first Democratic debate. Her team is even calling it the best day of the campaign so far.
And Sanders didn't fare too badly either. He received $1.3 million in campaign donations in the four hours after the debate started, according to the Washington Post.
Clinton and Sanders took to the stage with Martin O'Malley, Jim Webb and Lincoln Chafee to tackle issues ranging from gun control to climate change as CNN's Anderson Cooper moderated the debate.
Despite Sanders leaping to Clinton's defense over her email scandal, the evening wasn't short of clashes. They went head-to-head over U.S. involvement in the Middle East, gun control and economic policy, among other issues.
When Clinton was asked if she thought Sanders had been tough enough on gun control legislation, she responded: "No, I do not."
But Sanders defended his mixed gun control record, calling for better mental health services and stricter background checks.
The two also scrapped over foreign policy as Clinton, who voted for the 2002 invasion of Iraq, defended her judgment on foreign affairs.
Sanders said the Iraq war was "the worst foreign policy blunder in the history of our country."
Chafee also slammed the call, saying: "If you're looking at someone who made that poor decision in 2002 to go into Iraq when there was no real evidence of weapons of mass destruction, which I knew because I did my homework, that’s an indication of how someone will perform in the future. And that’s what’s important."
Clinton responded: "I recall very well being on a debate stage about 25 times with then-Senator Obama debating this issue. After the election, he asked me to be Secretary of State. He valued my judgment. I spent a lot of time with him in the situation room going over some very difficult issues."