He also thanked his political mentor, former president Alvaro Uribe, who will be sworn in as senator in August and is expected to lead the opposition to the government.
There were no reports of major problems or irregularities, says the BBC's Arturo Wallace in Bogota.
Nearly 48% of the 33 million Colombians registered to vote took part in the election 生意周轉.
Despite concerns in Colombia about high crime and unemployment, the campaign focused on the peace negotiations with the Farc, launched by Mr Santos in November 2012.
The talks, which are being held in Cuba, are aimed at ending five decades of conflict.
Mr Santos expects a deal to be signed by the end of the year, with the Farc giving up their armed struggle and joining the legal political process.
Government negotiators and rebel leaders have held several rounds of talks in the Cuban capital, Havana.
They have agreed on three points of the agenda drawn up in 2012 - land reform, future political participation and drug trafficking, which is allegedly the main source of income for the rebels 醫療用品 .
Three other points remain to be agreed on - the rights of the victims, disarmament of the rebels and the implementation of the agreement 耳穴戒煙 .