John Hume, an apostle of nonviolence who led Northern Ireland’s largest Catholic political party and helped quell three decades of bloodshed in the British province, bringing him international acclaim and a Nobel Peace Prize, died Aug. 3 at a nursing home in the border city of Londonderry. He was 83.

He had dementia, his family said in a statement announcing the death.

As the longtime head of the Social Democratic and Labour Party, Mr. Hume sought a unified Ireland, free of British rule. He ultimately championed Northern Irish self-determination, with fair treatment of all the province’s citizens, while seeking to end the bloody conflict known as the Troubles, which began in the late 1960s after the region’s Catholic minority protested discrimination by the Protestant majority.

Over the next 30 years, more than 3,500 people were killed and tens of thousands injured in riots, bombings, assassinations and politically motivated “disappearances.” The streets of Belfast and Derry, as Mr. Hume and other Irish nationalists called his hometown, were walled off and became battlegrounds for state security forces and paramilitary groups.

Self-described loyalists and unionists, primarily Protestant, fought to maintain Northern Ireland’s attachment to the United Kingdom; nationalists and republicans, including Catholic members of the Provisional Irish Republican Army (IRA), wielded Armalite assault rifles in the name of unification with the Republic of Ireland to the south.

While Mr. Hume sympathized with the aims of militant republicans, he insisted that Northern Ireland’s political future would be determined by nonviolent demonstrations, not bombs and bullets.

“There’s this historic, romantic Irish thing that the only way that you can solve the Irish problem is by spilling blood, the sacrifice syndrome,” he told the Catholic magazine Commonweal in 1984. “I say Ireland is a country to live for, not die for. I don’t want the young people dying for Ireland, I want them to live for it.”

Mr. Hume, a former schoolteacher who had once studied to become a priest, was widely considered Northern Ireland’s most influential peace activist. He traveled between Dublin, London and Washington to win support for the peace process; held secret negotiations with Gerry Adams, president of the IRA’s political wing, Sinn Fein; and received much of the credit for the Good Friday Agreement of 1998.

The agreement called for the IRA to set aside its weapons and established a power-sharing government based at Stormont Castle near Belfast. Months after it was signed, the Nobel Committee in Oslo awarded the Nobel Peace Prize to Mr. Hume and David Trimble, the Protestant leader of the Ulster Unionist Party, “for their efforts to find a peaceful solution to the conflict in Northern Ireland.”

While political observers credited Adams and other leaders with helping to forge the agreement, the committee cited Mr. Hume as “the clearest and most consistent of Northern Ireland’s political leaders in his work for a peaceful solution.” Interviewed after the Nobel announcement, Adams told the New York Times that “there would be no peace process but for his courage and vision.”

On a visit to Derry in 1995, President Bill Clinton called Mr. Hume “Ireland’s most tireless champion for civil rights and its most eloquent voice of nonviolence.”

The Good Friday Agreement came after years of struggle for Mr. Hume, who waded through tear gas as a fledgling politician and once tried to defuse a standoff between demonstrators and British paratroopers, who responded by hosing him with purple dye and arresting him. As the peace negotiations continued, his home was firebombed, his car burned out.

“Ireland is not a romantic dream; it is not a flag; it is 4.5 million people divided into two powerful traditions,” he said during the peace process. “The solution will be found not on the basis of victory for either, but on the basis of agreement and a partnership.”

The oldest of seven children, John Hume was born in Londonderry on Jan. 18, 1937. His father was a shipyard laborer who found it difficult to hold down a job after World War II because of discrimination against Catholics, and his mother worked in a shirt factory to support the family.

Mr. Hume received a government scholarship that enabled him to attend secondary school and was a seminarian for three years before changing course. He studied French and history and received a master’s degree from St. Patrick’s College in Maynooth, near Dublin.

Returning to Derry, he taught at a Catholic grammar school, organized credit unions and in 1960 married Patricia Hone, who gave up her teaching career to support Mr. Hume’s political work. She and their five children survive him.

“I’ve often said I’ve been a parcel and Pat delivers me,” Mr. Hume told the Guardian in 2001. “We are a team.”

Mr. Hume was elected to the Northern Ireland Parliament in 1969, helped found the SDLP a year later and became the party’s leader in 1979. He was later elected to the European Parliament and British Parliament, where he established himself as a lonely voice for Irish nationalism.

Mr. Hume was credited with shaping initiatives such as the 1973 Sunningdale Agreement, which formed a brief power-sharing government between unionists and republicans; the 1983-84 New Ireland Forum, which brought together the major parties of Northern Ireland to discuss a peace process; and the 1985 Anglo-Irish Agreement, which gave Dublin an advisory role in Northern Irish affairs.

Three years later, he secretly met with Adams at Clonard Monastery in Belfast, beginning the talks that laid the groundwork for an IRA cease-fire. When those negotiations became public a few years later, Mr. Hume was lambasted by loyalists and members of his own party. Adams was allegedly a former IRA commander, which the Sinn Fein president denied, and some critics accused Mr. Hume of bringing a terrorist into the fold.

While Mr. Hume’s negotiations with Adams were pivotal to the peace process, they were also linked to growing support for Sinn Fein, which now has more seats in the Northern Ireland Assembly than Mr. Hume’s SDLP.

Mr. Hume receded from the political scene after the passage of the Good Friday Agreement. His party deputy, Seamus Mallon, who died in January, served alongside Trimble as co-head of the new Northern Irish government, and in 2001, Mr. Hume retired from the SDLP leadership, announcing his full retirement from politics three years later.

The Good Friday Agreement has been tested by sporadic violence, including a 2019 attack in Derry that resulted in the death of a journalist. But on the whole, Mr. Hume was optimistic about a lasting peace.

“All conflict is about difference, whether the difference is race, religion or nationality. . . . Difference is of the essence of humanity,” he said in his Nobel lecture. “Difference is an accident of birth, and it should therefore never be the source of hatred or conflict. The answer to difference is to respect it. Therein lies a most fundamental principle of peace — respect for diversity.”