South Africa’s highest court on Monday ruled that former President Jacob Zuma was not eligible to serve in Parliament, a decision that may deepen political turmoil in the country just over a week before a crucial national election.

The decision threatens the political future of the 82-year-old Mr. Zuma, a former anti-apartheid hero who once led the liberation party, the African National Congress. Mr. Zuma had a bitter falling out with the A.N.C. last year after announcing he was supporting a new political formation.

The Constitutional Court, overturning a special electoral court’s earlier decision, ruled that Mr. Zuma could not stand as a candidate in the May 29 election because of a past criminal conviction.

Even though he cannot serve in Parliament, Mr. Zuma’s face will still appear on the ballot next to his new party, uMkhonto weSizwe, or M.K., because he is registered as its leader, according to the Electoral Commission of South Africa. He will, however, be removed from the list of candidates nominated to represent the party in the National Assembly, the commission said.

Mr. Zuma is a populist figure who attracts a devoted following, and his image could be enough to lift his party’s fortunes and hurt the A.N.C. The A.N.C. is fighting to maintain the absolute majority it has held since the start of South Africa’s democracy 30 years ago.

The M.K. party denounced the court’s decision in a statement on Monday, calling the court’s justices “10 highly compromised and conflicted individuals” who have too much power under the country’s constitutional democracy.

The party encouraged its supporters to give the party the majority it needs to overhaul South Africa’s government. M.K. said that if it won two-thirds of the 400 seats in South Africa’s Parliament, a nearly impossible feat for the fledgling party, it planned to install Mr. Zuma as president of the country.

“This heavily flawed and conflicted judgment is not the end but rather a pivotal moment affirming that the M.K. Party is the right choice for the Black poor and downtrodden,” the party said.

Some analysts said that sense of grievance could be a boon to Mr. Zuma and his party.

“I think he’s going to ride on that and leverage it in terms of swaying support to his side,” said Hlengiwe Ndlovu, a senior lecturer in the School of Governance at the University of the Witwatersrand in Johannesburg.

Mr. Zuma resigned from the presidency in 2018 amid widespread protests over allegations of sweeping corruption within his government. Three years later, he was convicted and sentenced for failing to testify at a public inquiry on corruption.

Mr. Zuma’s attempted political comeback has created a big test for South Africa’s young democracy.

He became the first former president to serve prison time in post-apartheid South Africa after his arrest in July 2021, though he was released on medical parole just two months into his 15-month sentence. The Constitutional Court later overturned his medical parole, but Mr. Zuma then received a presidential pardon from his successor-turned-political rival, Cyril Ramaphosa.

The court’s decision hinged on the length of Mr. Zuma’s sentence. While he was granted a remission that reduced his time in prison, he had been sentenced to 15 months, which made him ineligible to run, the court decided.

According to South African law, a person who has been convicted of an offense and sentenced to more than 12 months in prison cannot serve in the National Assembly. That applies to Mr. Zuma, Justice Leona Theron said, delivering the court’s decision.

Mr. Zuma is not “eligible and not qualified” to stand for election until five years after the completion of his sentence, the justice added.

During a marathon court appearance on May 10, Mr. Zuma’s lawyers tried to force the Constitutional Court justices to recuse themselves, arguing that the same justices who had sentenced him were ruling on his eligibility for Parliament. That argument was dismissed.

Tembeka Ngcukaitobi, a lawyer representing the electoral commission, which had opposed Mr. Zuma’s candidacy, argued that the law barring those convicted of a crime from running for a seat in the National Assembly “served to protect the public from lawbreakers now putting themselves up as lawmakers.”

Mr. Zuma’s decision to lead and campaign for an opposition party had deeply unsettled South African politics. Founded in December, uMkhonto weSizwe has quickly become one of the most visible opposition organizations in an election in which a record 52 parties are vying for votes on the national ballot.

South Africans vote for a party instead of an individual, but M.K. appears to be banking on the appeal of a familiar face: Mr. Zuma’s image is all over its campaign posters and T-shirts.

The party has already gained a foothold in the KwaZulu-Natal Province, Mr. Zuma’s traditional stronghold. Polls show that Mr. Zuma’s party could play kingmaker in a coalition government in the province.

The party has attracted voters who are aggrieved by the governing A.N.C., but it has also eaten into the support of smaller opposition parties who have struggled to capitalize on the A.N.C.’s waning support.

Over the weekend, Mr. Zuma took his campaign to Soweto, once the heart of A.N.C. support in Johannesburg, South Africa’s largest city, where his supporters filled a soccer stadium. Despite the legal setback, members of the M.K. party vowed to continue their campaign.

“It was a shock,” said Lebogang Moepeng, a senior member of the M.K. party. After its win at the Electoral Court, the party was confident the Constitutional Court would find in its favor, he said, adding that the party was prepared for these challenges.

“It would have been naïve of us to launch a party and not take stock of the risks,” Mr. Moepeng said. “Legal, political and otherwise.”

Mr. Zuma’s arrest and incarceration in 2021 set off deadly riots, and observers fear that his barring from Parliament could again lead to violence. But on Monday, the M.K. party urged its members to remain calm and refrain from violence.

“Of course there’s going to be mass discontent around it,” because many will see the ruling as a sign that the courts are biased in favor of the A.N.C., said Moshibudi Motimele, a lecturer in political studies and governance at the University of the Free State in South Africa.

If people believe the courts are not independent, she added, they may think, “‘We have to deal with things ourselves.’ And in South Africa, ‘deal with things ourselves’ means violence.”

But Mr. Ramaphosa, the current president and incumbent, said in a radio interview, “I am not concerned about this instigating violence.” He added, “President Jacob Zuma should be the one person who should respect the rule of law.”

John Eligon contributed reporting from Johannesburg.