His publisher said he passed away at a clinic in Luebeck on Monday morning.
Born in what was then Danzig, Grass served in the German military in World War Two and published his breakthrough anti-Nazi novel, The Tin Drum, in 1959.
Later in life he became a vocal opponent of German reunification in 1990, and argued afterwards that it had been carried out too hastily.
The German Foreign Minister Frank-Walter Steinmeier was “deeply dismayed” to hear of the author’s death, the foreign ministry tweeted.
Writer Salman Rushdie described Grass as “a true giant, inspiration, and friend.”
Grass’s home town became the Polish city of Gdansk after the war; he spent much of his later life living near Luebeck.
Many of his writings focused on the Nazi era, the horrors of the war, and the destruction and guilt that remained after Germany’s defeat.
Germans were shocked when he revealed in his 2006 memoir Skinning the Onion that as a teenager he had volunteered to join the army and had served in the Waffen-SS – the combat arm of Hitler’s dreaded SS paramilitary force, which was responsible for atrocities throughout Nazi-occupied Europe.
Previous accounts of his life had suggested he had been an anti-aircraft gunner and had been conscripted into the military.