The court, which is led by a former member of the MPLA, made an initial ruling on Monday against a complaint filed by UNITA but Thursday's decision is final and cannot be appealed, paving the way for Lourenco to be sworn in next week.
After the country's most closely fought election, the electoral commission declared last week the People's Movement for the Liberation of Angola (MPLA) the winner, prolonging its nearly five decades of uninterrupted rule and handing President Joao Lourenco a second term.
Just over 51% of voters had supported the ex-Marxist MPLA, it said. The National Union for the Total Independence of Angola (UNITA), its longtime opponent and former civil war enemy, took about 44%, its best result on record, according to the commission.
UNITA leader Adalberto Costa Junior rejected the results, citing discrepancies between the commission's count and the party's own tally. He accused the electoral commission, which mostly controlled by the MPLA, of fraud.
According to UNITA's parallel count, it got 49.5% of the vote and the MPLA 48.2%. A parallel count by civic movement Mudei, which monitored the process, also showed UNITA slightly ahead.
UNITA and other opposition parties called for peaceful demonstrations to protest against what they have described as election "irregularities".
Among other issues, there were only 1,300 observers to cover a country twice the size of France, around 2.7 million deceased people were included in the electoral roll, and the MPLA was given more air time than others
People took to social media to urge the electoral commission to release its result sheets so they can be compared with parallel counts.
The commission has repeatedly said the process was fair and transparent.
Angola has placed its armed forces in "high combat readiness" until Sept. 20 to avoid incidents that might "disturb public order", it said.
Analysts fear the court decision could ignite mass street protests among a poor and frustrated youth who voted for UNITA.