“Some have told us to mind our own business,” Mr. Ramaphosa said in a nationally televised address after the court issued its order. “Others have said it was not our place. And yet it is very much our place, as people who know too well the pain of dispossession, discrimination, state-sponsored violence.”

Mr. Ramaphosa has been under pressure both abroad and at home. His foreign policy has often been seen as thumbing his nose at crucial Western allies, his approach to domestic issues criticized as indecisive. Israel and others have also accused South Africa of hypocrisy, arguing that it had shielded suspects in the genocide in Rwanda. (South Africa did eventually extradite a prime suspect to face charges in Rwanda.)

Getting an outcome that his government had sought at this stage of the case could allow Mr. Ramaphosa to fashion himself as a leader whose midsize nation had achieved outsized results.

“The International Court of Justice has vindicated us,” Mr. Ramaphosa said.

South Africans have long equated the plight they faced under apartheid to what Palestinians are experiencing under blockade in Gaza.

As Mr. Ramaphosa and top officials from his party, the African National Congress, watched the court deliver its judgment, they clapped at every word that seemed in their favor. When the order was delivered, they broke out in hymns from the days of the fight against apartheid.

They also serenaded Ronald Lamola, the South African justice minister who led the legal team that argued the case at the court several weeks ago. “Free! Free! Palestine!” they chanted.

“I’m very humbled, and I believe that Mandela will be smiling in his grave that we stood on his shoulders and we did him very proud,” Mr. Lamola said, referring to Nelson Mandela, who had said that South Africans would not be free until Palestinians were.

In a nation where many are divided, economic hardship is widespread and basic services like electricity are breaking down, frustration is high with the ruling African National Congress, which has governed South Africa since the end of apartheid in 1994.

Support for Palestinians has largely been a unifying issue in South Africa. But the case could cause challenges for Mr. Ramaphosa and his party.

South Africa has a small but robust Jewish community, and some have been highly critical of the government’s stance on Israel, even accusing the government of antisemitism. Jews played a significant role in the anti-apartheid fight and the efforts to build the democratic nation, and some Jewish South Africans have said they feel betrayed.

The Democratic Alliance, the lead opposition party, has not been critical of the case itself, but has said that the government has been selective in its condemnation of human rights violations, weakening South Africa’s credibility across the globe.

A bipartisan group of 210 U.S. congressional members sent a letter this week to Secretary of State Antony J. Blinken expressing “our disgust at this filing” of the genocide case, saying that it “perpetrates false and dangerous allegations against the Jewish state.”

U.S. lawmakers are set to consider the renewal of a trade deal important to South Africa, one that had seemed in jeopardy last year. Relations between the two nations had reached a low point then, after the U.S. ambassador in South Africa accused South Africa of providing weapons to Russia for its war in Ukraine.

But officials from both nations engaged in concerted talks and had appeared to patch up their differences. The question now is whether fresh wounds will open over the war in Gaza.

South African officials have brushed aside criticism of their efforts in the international court, saying that they are following proper procedures that are meant to settle these sorts of disputes. “It is a victory for international law,” Mr. Lamola said of the court judgment.

South Africa faces national elections later this year. Sithembile Mbete, a senior lecturer in political science and international relations at the University of Pretoria, said it would be “a bit simplistic” to view the president’s pursuit of the case as a campaign ploy — but the outcome could still help his party with voters.

“I think this is a huge win for the A.N.C. in government and the Ramaphosa administration in terms of how the country has projected itself externally,” Dr. Mbete said.