Festus G. Mogae, a former president of Botswana who was celebrated for leading that diamond-rich African nation out of a searing H.I.V. and AIDS crisis, in part by embracing a flagship American aid program, died on Friday in Botswana. He was 86.

His death was announced by the country’s current president, Duma Boko, who said in a televised address that Mr. Mogae “carried out the functions of this office with dignity, with finesse and with amazing humility.”

Mr. Boko did not specify the cause of death or say where in Botswana he died. Mr. Mogae’s office announced last month that he had been admitted to a hospital in Gaborone, the country’s capital, but had returned home and was recuperating.

Mr. Mogae was elected the third president of independent Botswana in 1998, and remained in office until 2008. (A constitutional amendment passed in 1997 limited the president to two five-year terms.)

He and his administration made it a priority to tackle H.I.V., which by the early 2000s had infected about 40 percent of Botswana’s adult population. Mr. Mogae said the country’s citizens were “threatened with extinction” by the disease.

During his time in office, Botswana became the first African nation to provide its citizens with free antiretroviral treatment through the U.S. President’s Emergency Plan for AIDS Relief, or PEPFAR.

By 2021, almost all H.I.V.-positive people in the country were on lifesaving treatment and virally suppressed. (PEPFAR, which supports testing and treatment and was started under President George W. Bush in 2003, has been targeted for severe cuts by President Trump.)

Festus Gontebanye Mogae was born on Aug. 21, 1939, to cattle farmers in the town of Serowe in central Botswana. His father served as a village headman, or local leader.

An economist by training who studied at Oxford and the University of Sussex in the United Kingdom, Mr. Mogae joined Botswana’s civil service in 1968, two years after the country gained independence from Britain.

He worked for the International Monetary Fund and the Bank of Botswana and served as finance minister from 1989 to 1998 and vice president from 1991 to 1998. As president, Mr. Mogae was credited with overseeing substantial growth in Botswana’s economy, propelled by diamond production.

He married Barbara Modise in 1967. She survives him, along with their three daughters, Nametso, Chedza and Boikaego.

In 2008, Mr. Mogae was awarded the prestigious Ibrahim Prize for Achievement in African Leadership, which is given to former heads of state to recognize good governance and respect for term limits. He later served as a special U.N. envoy for climate change and on nonprofit and for-profit boards.

When he announced in 2007 that he would be leaving office the next year, Mr. Mogae said, “I look forward in the next nine months to retirement and rest.”

“I do so in the conviction that I did my best,” he added. “Like Frank Sinatra, I did it my way, and like Tony Blair, I did what I thought was right.”