Grass was asked why he was making the disclosure after so many years during an interview with the Frankfurter Allgemeine Zeitung, in which he discusses a memoir about the war years to be published next month.
"It weighed on me," he said. "My silence over all these years is one of the reasons I wrote this book. It had to come out, finally… Later this feeling of shame burdened me."
The Frankfurter Allgemeine Zeitung ran excerpts of the interview on its website, in advance of a fuller version in the newspaper today.
Grass, 78, is regarded as the literary spokesman for the generation of Germans who grew up in the Nazi era and survived the war. He won the Nobel Prize for literature in 1999 for works such as the 1959 novel "The Tin Drum," made into an Oscar-winning film in 1979.
He has long been active in left-wing politics as a sometimes critical supporter of the Social Democratic Party, and is regarded by many as a moral voice who has opposed xenophobia and war.
The article quoted him as saying he had originally volunteered for Germany's submarine service at age 15 but was not accepted, only to be called up at 17 to the Waffen SS 10th Armored Division Frundsberg, in Dresden.
Grass said he volunteered for military service to escape the confinement he felt as a teenager in his parents' house. He had been in the Arbeitsdienst, a force of laborers helping the military.
"It happened as it did to many of my age," he said. "We were in the labor service, and all at once, a year later, the call-up notice lay on the table. And only when I got to Dresden did I learn it was the Waffen SS."
Concentration camps and mass executions
The SS — Schutzstaffel, or "protective echelon" - started as a small bodyguard unit for Hitler headed by Heinrich Himmler.
It later became a huge organization that ran concentration camps and carried out mass executions of political opponents, Jews, Gypsies, Polish leaders, communists, anti-Nazi guerrillas and Soviet POWs.
It included the Waffen SS, a combat force that fought alongside units of the regular army and gained a reputation as fanatical.
The SS was declared a criminal organization by the Nuremberg war crimes tribunal after the war.