The win will boost Clinton's momentum ahead of Super Tuesday, which sees 865 delegates up for grabs
Hillary Clinton has won the South Carolina Democratic primary over rival Bernie Sanders in a victory which propels her into next week's Super Tuesday voting in 11 states.
The win is Mrs Clinton's third in the first four Democratic contests and cements her status as the frontrunner to take the party's nomination for the 8 November presidential election.
She earlier won Iowa by a narrow margin, and also took victory in Nevada. Vermont Senator Mr Sanders was victorious in the New Hampshire primary.
The result in South Carolina establishes Mrs Clinton's strength among black voters, a crucial Democratic constituency who make up more than half the party's primary electorate in South Carolina.
Addressing her supporters in South Carolina, Mrs Clinton said: "Thank you so much, from one end of this state to another.
"I am so greatly appreciative because today you sent a message. In America, when we stand together, there is no barrier too big to break.
"We've now gone through four early states and I want to congratulate Senator Sanders on running a great race, and tomorrow this campaign goes national.
"We are going to compete for every vote in every state. We are not taking anything, and we are not taking anyone, for granted."
She also wrote on Twitter: "To South Carolina, to the volunteers at the heart of our campaign, to the supporters who power it: thank you."
Facing tough odds to win in the state, Mr Sanders spent most of the past week in states that will vote in March.
As the results rolled in on Saturday, he was scheduled to hold a rally in the Super Tuesday state of Minnesota.
"Let me be clear on one thing tonight. This campaign is just beginning," Mr Sanders said in a statement.
"We won a decisive victory in New Hampshire. She won a decisive victory in South Carolina. Now it's on to Super Tuesday."
There are 865 Democratic delegates up for grabs in the Super Tuesday contests in 11 states and American Samoa.
Mr Sanders is hoping to stay close to his rival in the South while focusing most of his attention on states in the Midwest and Northeast, including his home state of Vermont.