“We are going to compete for every vote in every state. We are not taking anything, and we're not taking anyone, for granted,” Clinton told a cheering crowd in Columbia, South Carolina.
With 100 percent of polls reporting, Clinton captured 73 percent of the vote, while Bernie Sanders' share was 26 percent.
According to exit poll results, Clinton was boosted by overwhelming support among black voters. Blacks accounted for 62 percent of South Carolina Democratic primary voters and Clinton won 84 percent of their votes. In fact, Clinton won blacks by a wider margin than Obama did in 2008.
"Today, you sent a message in America: When we stand together, there is no barrier too big to break," the former secretary of state said.
"Let me be clear on one thing tonight. This campaign is just beginning,” Sanders said. “We won a decisive victory in New Hampshire. She won a decisive victory in South Carolina. Now it’s on to Super Tuesday.”
The Vermont senator added a note of confidence: "Our grassroots political revolution is growing state by state, and we won't stop now."
Clinton also congratulated Sanders, saying: "We’ve now gone through four early states, and I want to congratulate Senator Sanders on running a great race."
“Despite what you hear, we don't need to make America great again,” she said, referring to Trump’s campaign slogan. “America has never stopped being great. But we do need to make America whole again."
The ABC News Analysis Desk contributed reporting.