Roy L. Prosterman, a lawyer who left a lucrative corporate law practice to champion land reform in the underdeveloped world, died on Feb. 27 at his home in Seattle. He was 89.

His death was announced by the Seattle land-rights institute Landesa, of which he was a founder. The organization did not specify a cause.

Mr. Prosterman worked with governments in some 60 countries in Asia, Africa and Latin America over nearly six decades, crafting plans to give a degree of ownership to peasant families. Sometimes the governments he worked with obtained land by expropriating large tracts, with compensation to the owners. At other times, the government simply gave away land it owned.

Seeing land rights as the key to lifting up the world’s millions of rural poor people, he pushed authoritarian governments in places like Vietnam and El Salvador, as well as emerging democratic ones in countries like India, to distribute farmland to impoverished farmers.

In an obituary, Landesa said that millions of people had benefited from the programs created by Mr. Prosterman and his group. Landesa, which was founded in 1981 as the Rural Development Institute at the University of Washington and became an independent organization in 1992, was “an early, and often lonely, voice recognizing the importance that access to land and security of land has in uplifting the lives of the poor in agrarian economies,” the Nobel-winning economist Joseph Stiglitz wrote in the preface to “One Billion Rising: Law, Land and the Alleviation of Global Poverty” (2009), a book edited and partly written by Mr. Prosterman.

For Mr. Prosterman, the son of a Russian immigrant, the epiphany came early in his career. As a young Harvard Law School graduate, he landed a job at one of the most prestigious of New York’s white-shoe law firms, Sullivan & Cromwell. In 1963 the firm sent him to the impoverished West African nation of Liberia for a client looking to build a large port there.

“The quarters that he and his colleagues in the corporate law firm were staying in were quite luxurious,” the rural development expert Tim Hanstad, his partner and co-founder of Landesa, recalled in an interview.

“They were eating imported caviar and salmon from Norway,” Mr. Hanstad said, while the waterfront slums of Liberia’s capital, Monrovia, are among the most desperate in West Africa: muddy, crowded, with little access to sanitation or running water.

“It was a very sobering experience discovering how badly many people on the planet live,” he said in a speech at Claremont McKenna College in 2006, when he received the Henry R. Kravis Prize in Nonprofit Leadership. The conditions, he said, were “beyond the point of poverty that would describe most of the world’s poor.”

Dissatisfied, he left the law firm in 1965 to teach property, antitrust and international investment law at the University of Washington, already consumed by the idea of using his training to aid the world’s rural poor. “He was looking to live a life of purpose, of greater purpose,” Mr. Hanstad said.

A student pointed him to a law-review article suggesting uncompensated expropriation as a tool for land redistribution in Latin America; Mr. Prosterman surmised that “if you tried to solve it that way you would likely end up with civil war instead of land reform,” he told The New York Times in 2012.

In 1966 he wrote a counterproposal in Washington Law Review titled “Land Reform in Latin America: How to Have a Revolution Without a Revolution.” He insisted that “the view that land reform should be carried out with less-than-full compensation of the landlords must be discarded.”

The U.S. Agency for International Development noticed and sent him to South Vietnam in the middle of the Vietnam War as part of the attempt to woo peasants away from the surging Vietcong. Mr. Prosterman came up with a “land to the tiller” law, pushed by President Nguyen Van Thieu through Vietnam’s National Assembly, which in 1970 gave ownership to hundreds of thousands of tenant farmers in return for a “decent price,” Mr. Prosterman recalled in the 2012 interview. He would often note that as a result of the law, rice production surged and rural recruitment by the Vietcong plummeted.

Mr. Prosterman was widely recognized for the Vietnam land law, which a New York Times editorial called “probably the most ambitious and progressive non-Communist land reform of the 20th century.” It became his calling card. But it was not enough to save the Thieu government.

For Mr. Prosterman, the accomplishment led to assignments in El Salvador and elsewhere. Mostly, he did not expound world-transforming visions. Land reform, he said in the 2012 interview, “simply puts a given population — present or future — into a relationship with that land base that is most productive and equitable.”

The outcomes in El Salvador were mixed, as they had been in Vietnam; again Mr. Prosterman was called in by the Agency for International Development, in 1980, in the midst of a civil war between leftist guerrillas and a right-wing government supported by the United States. Mr. Prosterman noted in a New York Times guest essay in February 1981 that both left and right hated the land project he had helped with. Nonetheless, he wrote optimistically, “40 percent of all cropland has been transferred to more than 210,000 peasant families.”

But in May of the next year, the New York Times correspondent Raymond Bonner wrote, “In less than one month as a legislative body, El Salvador’s Constituent Assembly has blocked most of the country’s land redistribution effort from being carried out.” Today, Landesa’s website simply notes that El Salvador’s land reforms “had some limited successes at addressing inequality.”

In more recent decades Mr. Prosterman focused much of his effort on India, which he said in 2012 had “the highest concentration of poor people on the planet.” He pushed what he called “new generation” ideas, in which India’s state governments would give “microplots,” a tenth of an acre or less, to landless people, with “women’s names jointly on the title as owners.”

In one of the last things he wrote, in 2009, Mr. Prosterman acknowledged that “little scope remains for traditional land-to-the-tiller programs that use expropriatory methods to obtain private land” to give farms to tenant farmers. This was, paradoxically, largely because of the decline of “authoritarian” governments, whose existence had made large-scale expropriation easier.

“When the power distances are so great” between landlord and tenant, “democracies don’t work well,” Mr. Hanstad explained.

Roy L. Prosterman (the “L” did not stand for anything) was born on July 13, 1935, in Chicago, the only child of Sidney Prosterman and Natalie (Weisberg) Prosterman. His father was a businessman. He graduated from South Shore High School at 16 and from the University of Chicago with a Bachelor of Arts at 18 in 1954. He received his law degree in 1958.

Mr. Prosterman and his international partners or the organization he founded received a number of awards, including the Gleitsman Foundation International Activist Award for alleviating inequality in 2003, the Schwab Foundation Outstanding Social Entrepreneur award in 2002, and the University of Chicago Public Service Award in 2010.

No immediate family members survive.

During his career, Mr. Prosterman was careful to downplay the political ramifications, as opposed to the human ones, of his work.

“The very fact of giving people secure rights to at least some small sliver of the earth’s surface,” he said in 2012, “strongly motivates them to make improvements that increase production and allow the family to make a number of basic-needs investments.”