Osmin Tobar and his brother J.R. were seven and two years old when they were picked up by officials in a poor district of Guatemala City in 1997, ostensibly for having being abandoned.
Tobar was adopted by a family in the US city of Pittsburgh. His brother suffered a similar fate, although his whereabouts are unknown.
"On behalf of the state... I apologize publicly for the events of which you were victims," President Bernardo Arevalo said at an event in Guatemala City.
The state's role in the incident "has no justification," he added.
Tobar welcomed the apology, which he said "recognizes past mistakes and signals a commitment to justice and integrity."
"This apology is more than symbolic -- it recognizes the pain endured by those affected and will pave the way for healing and progress," the 34-year-old added.
Tobar, who has a wife and a son, said that the loss of a sense of identity had led him to become addicted to drugs and alcohol.
He said that he was sharing his story "to raise awareness about the darkness of human trafficking, advocate laws that focus on the preservation of families, and offer hope to survivors in the shadows."
His mother, Flor Ramirez, said that she had been devastated by the separation from her sons.
"As parents we suffer, but those who suffer the most are our children because they are taken to a strange place, with a strange family," she said.
Tobar, who now lives in Guatemala, has become the voice of nearly 28,000 children from the Central American nation who were victims of such adoptions in the 1990s and 2000s.
His was the first such case heard by the Inter-American Court of Human Rights, and led to a ruling against Guatemala in 2018 that paved the way to the official apology.
Before Guatemala moved to end the practice in 2007, about 5,000 children were put up for adoption each year, mostly by American couples who paid about $50,000, according to human rights groups.
In total, the adoptions were estimated to generate around $250 million a year.
