Schwarzbaum died on Sunday night, according to Christoph Heubner, executive vice president of the IAC.
“His death represents a great loss to the collective memory. We will all miss his anger and humanity,” Heubner said.
Schwarzbaum testified in 2016 against former Auschwitz camp guard Reinhold Hanning, who was sentenced to five years in prison but died a few months after the verdict before he could go to jail.
In late 2021, Schwarzbaum also appeared as a witness in the trial of Josef Schütz, a 101-year-old former guard at the Sachsenhausen camp.
He had been due to participate in a further hearing of that trial this week, Thomas Walther, a lawyer specialising in Nazi war crimes, told AFP.
In a written statement due to be read by Walther, Schwarzbaum had planned to ask the accused to “tell us the historical truth”.
“Speak here in this place about what you experienced — as I have done for my part,” he wrote.
Schwarzbaum had often expressed anger and regret that so few Nazi war criminals had been brought to justice, especially in Germany.
He “did not want hatred, he wanted justice”, the IAC said in a statement.
Schwarzbaum was born in 1921 into a Polish Jewish family in Hamburg but grew up in Bedzin, Upper Silesia, in present-day Poland before his family was deported to Auschwitz in 1943.
He was the only member of his family to survive the camps of Auschwitz, Buchenwald and a sub-camp of the Sachsenhausen complex north of Berlin.
He later worked in Berlin as an art and antiques dealer, while also campaigning tirelessly to keep the memory of the Holocaust alive through lectures and talks around the world.
“It is with great sadness, respect and gratitude that Holocaust survivors all over the world bid farewell to their friend, fellow sufferer and companion Leon Schwarzbaum, who in the last decades of his life became one of the most important contemporary witnesses of the Shoah,” Heubner said in a statement.