Grass, 78, told the Frankfurter Allgemeine Zeitung newspaper that he joined the Waffen-SS when he was 17. He was bing interviewed on Saturday to discuss his autobiography, Peeling Onions.
The Waffen was the combat unit of Hitler's paramilitary SS organisation.
Grass had previously said he was drafted in 1944 to help anti-aircraft gun teams. He was held as a prisoner of war until 1946. After the war, he became an outspoken pacifist and icon of the German left.
He said the secret had been weighing on his mind and was one of the reasons he wrote the autobiography, which details his war experience.
The revelation has caused outrage in Poland, especially in the coastal city of Gdansk, the Baltic city where Grass was born and which has featured in several of his works.
The former Polish president, Lech Walesa, like Grass, hails from the city known as Danzig in German and also holds honorary citizenship.
On Monday he urged the author to give up the honorary title himself rather than wait for Gdansk officials to strip him of it.
"Who will talk to him here now or invite him?" he said. "I am happy we never met, that I never had to shake his hand. I lost my father in the war and Grass was in the SS."
Jacek Kurski, a member of parliament from Gdansk for the ruling Law and Justice party told a news conference that "it is unacceptable for a city where the first blood was shed, where World War Two began, to have a Waffen-SS member as an honorary citizen".
Kurski said his party would propose a resolution to the Gdansk city council to strip Grass of his honorary citizenship if the author failed to surrender it on his own.
"It would be good if Grass gave up the title voluntarily," he said.
Poles and Germans lived together in Gdansk before the war, but most of the German population fled as the war ended or were later expelled.
Grass is best known for his 1957 novel The Tin Drum and in 1999 was awarded the Nobel Prize in Literature.