"I pay homage to the Chadian people, a mature people, a people which has won by being politically mature. The people have made their choice and it's me," Mr Deby told journalists.
The veteran leader's victory was never in any doubt after opposition parties boycotted the polls claiming Mr Deby's regime was syphoning off oil revenues earmarked for the development of his desperately poor nation.
Four other virtually unknown candidates in the May 3 polls won between 3.7 and 8.8 percent of the vote.
Election Commission President Ahmat Mahamat Bachir said turnout was 61.5 percent of Chad's 5.7 million eligible voters, although independent observers described lacklustre participation.
Opposition spokesman have claimed the vote was effectively invalidated by "massive abstention", with voters refusing to legitimise an "electoral masquerade".
Two days after the election, an observer group with representatives of the African Union and 19 non-governmental organisations said the polls had been "free, open and fair," despite some "minor" organisational problems.
The AU, the United States, Chadian civil society and religious groups had pushed for a later polling date citing fears of fraud, rebel attacks and instability caused by the crisis in Sudan's neighbouring Darfur region.
The president pushed ahead with the polls despite an attempted coup against him by rebels who attacked pro-government forces in the capital N'Djamena just three weeks earlier.
"With this victory you have proved wrong those who said yesterday that Chad will explode and prove once again that democracy works in Chad and that it is progressing," said Mr Deby.
Mr Deby, 54, a former rebel leader who seized power in 1990 with the overthrow of Hissene Habre, was elected in 1996 after introducing a multi-party system and reelected five years later in a poll criticised by the opposition.
A constitutional amendment adopted last year allowed him to stand for a third term.