The court ruled in favour of Mnangagwa after Chamisa failed to prove his allegations of vote fraud.
He accused the Zimbabwe Electoral Commission (Zec) of irregularly declaring the incumbent, Emmerson Mnangagwa, the victor of Zimbabwe's first polls without former president Robert Mugabe.
According to the official results, Mnangagwa won almost 51 percent of the vote to defeat MDC leader Nelson Chamisa, who received just over 44 percent.
The opposition alleged that Mr Mnangagwa's 50.7% vote share - which was just 30,000 more votes than the minimum needed for outright victory - was suspicious.
After the ruling, the European Union issued a statement saying all parties should accept the verdict, adding that the new government needed to push through electoral reforms after a vote that had "revealed improvements as well as challenges".
Last month's election, the first since Robert Mugabe was removed as President in a November coup, was expected to pull Zimbabwe out of its diplomatic isolation, end worldwide sanctions and prompt an economic recovery.
The president has tweeted that he was "not surprised" by the Constitutional Court's ruling. "It is time to build our nation and move forward together".
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"There has been a massive doctoring of evidence", Thabani Mpofu, representing the MDC, told the court.
"We think it is incumbent upon all of us to say the election period is over and Zimbabweans must put their energies towards a process of nation-building", he said.
The Zimbabwe Electoral Commission declared Mnangagwa victor of the election after some delays, which some observers said created opportunities for citizens to speculate about the potential to manipulate results.
"Our unstinting belief in the rule of law means we shall not only respect the verdict of the bench but we shall also doggedly pursue all constitutionally permissible avenues to ensure that the sovereign will of the people is protected and guaranteed".
President Mnangagwa said the State could only function efficiently in a peaceful environment.
Mnangagwa took over from Mugabe after the long-time ruler was ousted following a short military takeover in November 2017 after 20 years as president.
Campaigning was more open than previous votes, but the election was marred by the army opening fire on protesters, killing six, allegations of vote-rigging and a crackdown on opposition activists.
He also said that if the opposition had requested a re-count of the vote it would have provided "a clear and indisputable picture of the outcome of the election".
A credible vote is key to lifting worldwide sanctions as the southern African nation tries to move away from the long shadow of Robert Mugabe's 37-year rule.