Rosa Roisinblit, an Argentine human-rights activist who fought to establish the truth about the fate of her pregnant daughter and thousands of others who were kidnapped by security personnel and “disappeared” during the country’s 1976-83 military dictatorship, died on Saturday in Buenos Aires. She was 106.

Ms. Roisinblit’s death, in a nursing home, was announced by the organization of which she was honorary president and an early member, the Grandmothers of Plaza de Mayo.

During the “Dirty War” waged by Argentina’s right-wing military dictatorship against perceived opponents, Ms. Roisinblit and others donned symbolic white head scarves and gathered weekly in that central square in Buenos Aires to protest, at great risk, the disappearance of their children.

Human rights groups estimated 30,000 Argentines were kidnapped, tortured, drugged, pushed from airplanes and never heard from again, in a “system of licensed sadism,” as the Oxford legal scholar Ronald Dworkin wrote.

Among the disappeared were Ms. Roisinblit’s 25-year-old daughter, Patricia, and her son-in-law, José Manuel Pérez Rojo, who were taken on Oct. 6, 1978, when Patricia was 8 months pregnant with a boy. Patricia and José had been members of the Montoneros, a left-wing guerrilla group, but both had reportedly left that life behind by the time of their abduction.

Their 15-month-old daughter, Mariana, initially abducted along with them, was quickly returned to the family by the military regime’s agents. Ignoring police warnings not to look for Patricia, Ms. Roisinblit filed a habeas corpus petition; it was rejected.

As Patricia’s due date approached, she also scoured hospitals and orphanages, she later recalled to the journalist and author Caroline Brothers. At one point, Ms. Roisinblit added, she encountered a judge friendly to the junta who told her that she “would never hand over a child to the likes of us.”

She later discovered from testimony that Patricia had been taken to a “maternity ward” at the Naval Mechanics School, a detention and torture center near Buenos Aires, and gave birth in captivity that November.

Her infant son was given to a civilian Air Force intelligence officer, Francisco Gómez, and his wife; the boy was one of reportedly hundreds of those born in clandestine torture centers who were, under false papers, transferred to childless military couples.

Ms. Roisinblit, a widow and retired obstetrician , was desperate. Soon after her daughter’s abduction, she joined a fledgling offshoot of the group of mothers that had started gathering on the Plaza de Mayo. This new group was the Grandmothers.

“I wanted to fight, I wanted to do something, I was in a rush to do something,” Ms. Roisinblit is quoted as saying in a new book about the grandmothers, “A Flower Traveled in My Blood: The Incredible True Story of the Grandmothers Who Fought to Find a Stolen Generation of Children,” by Haley Cohen Gilliland.

For 33 years, Ms. Roisinblit served as vice president of the grandmothers’ group, helping family members of the disappeared by rallying international support for their cause and following up on anonymous tips about wives of government officials who suddenly had children without being pregnant. They also kept up on the latest genetic testing methods, in the hope of one day identifying stolen kin.

Patricia, a medical student at the time of her abduction, was never seen again, nor was her husband, who owned and operated a toy store. Ms. Roisinblit was able to find her grandson, Guillermo, after her granddaughter, Mariana, a writer and academic, received a tip in 2000 that a fast-food worker in Buenos Aires might be related to her. Tests proved that they were.

According to the Grandmothers of Plaza de Mayo, some 300 “stolen grandchildren” remain to be found, and around 140 have been given back to their families.

In their statement announcing Ms. Roisinblit’s death, the Grandmothers organization said: “The greatest recognition she received is that of the grandchildren she found, who at every meeting embraced her as if she were their own grandmother.”

Rosa Tarlovsky was born on Aug. 15, 1919, to Salomón Tarlovsky, a farmer and rancher who went bankrupt during the Depression, and Ana (Milstein) Tarlovsky, in Moisés Ville, an agricultural community of Jewish immigrants who had fled pogroms in czarist Russia.

As a teenager, she moved to the city of Rosario to study obstetrics and graduated from the National University of Litoral. She moved to Buenos Aires in 1949 and married Benjamin Roisinblit, an accountant, in 1951. Patricia was born the next year.

She is survived by her granddaughter, Mariana Eva Pérez, and her grandson, Guillermo Pérez Roisinblit, who eventually changed his surname from Gómez after confronting the couple who raised him in what he described as an abusive home environment.

In 2005, Argentina’s Supreme Court lifted a blanket amnesty covering junta-era military officials, as the left-leaning President Néstor Kirchner sought a reckoning on human rights cases stemming from the Dirty War years. He formed a political alliance with the Mothers and Grandmothers groups.

At the age of 96, in the spring of 2016, Ms. Roisinblit had the satisfaction of facing in court the men charged with kidnapping her daughter and son-in-law: Francisco Gómez; Luis Trillo, a regional intelligence chief in the Air Force; and the Air Force commander Omar Graffigna, one of the last surviving members of the dictatorship’s inner circle.

The judge asked Ms. Roisinblit if she was prepared to tell the truth, Ms. Gilliland recounted in her book.

“I never stop telling the truth,” Ms. Roisinblit replied. “It’s a trait of mine.”

The judge asked her how many children she had. Ms. Roisinblit hesitated.

“I had a daughter. But I don’t anymore,” she answered.

Her daughter and son-in-law’s kidnappers were all found guilty and sentenced to prison: Mr. Graffigna and Mr. Trillo to 25 years, and Mr. Gómez, to 12 years.

“I need the Argentine state to tell me who took them,” Ms. Roisinblit said in court, at the end of her testimony. “Why it took them. Who gave them a trial. Who convicted them. Where are they?” she asked. “I want to find the remains of my children, because then I’d have a place to lay a flower.”