"We have determined that, without the shadow of a doubt, soap was produced using substances obtained from human bodies at the anatomical insitute of the Medical Academy of Danzig, led by Professor Rudolf Spanner," Paulina Szumera of the IPN told AFP.
Danzig is the German name for the Polish city of Gdansk.
"We launched our investigation to still the voices denying that this ever happened," she said.
Evidence for Nuremberg trials
For the IPN probe, Polish scientists studied a bar of soap that was presented as evidence during the Nuremberg Nazi war crime trials after World War II, that was in the archives of the International Court of Justice in The Hague, Szumera said.
Polish television station TVN24 cited IPN investigators as saying the bodies of prisoners at the Nazi concentration camp of Stutthof, in northern Poland, and at Gdansk municipal jail were used to make the soap.
The bodies of patients at a psychiatric hospital in Gdansk were also used, the investigators told TVN24.
Almond extract was added to the soap to give it a palatable scent.
Soaps are usually made from fats and oils that react with lye (sodium hydroxide).