Mugabe, who has been president since Zimbabwe won independence from Britain in 1980, won 61 percent of the votes while his opponent and current Prime Minister Morgan Tsvangirai could barely manage 34 percent votes.
Alleging that the elections for parliament and president were fraudulent, Tsvangirai earlier vowed to take legal action. He also said that his Movement for Democratic Change (MDC) party would no more work with Mugabe's Zanu-PF party.
The two parties have been working together in a coalition since 2008 election which sparked widespread violence.
Results from this week's parliamentary election showed the MDC had been trounced, winning just 49 seats compared with Zanu-PF's 158.
In a news conference before the presidential result was announced, Tsvangirai said Zimbabwe was "in mourning".
Tsvangirai said he would produce a dossier of the alleged electoral fraud and called on the southern African regional bloc, Sadc, to investigate.
The European Union, which maintains sanctions on Mugabe and his senior aides, said it was concerned about "alleged irregularities and reports of incomplete participation" in the election.
The largest group of domestic monitors the Zimbabwe Election Support Network (ZESN) had said problems with voter registration had left up to one million people unable to cast their ballots, most of them in MDC strongholds.
On Saturday, one of the nine members of the election commission resigned over the way the election was conducted.