Mr Steinmeier and other world leaders are in Poland to commemorate the outbreak of the conflict.
Speaking in the capital, Warsaw, Mr Steinmeier apologised for the "horrific war" unleashed by Germany.
"This war was a German crime," he said in a speech.
His Polish counterpart, Andrzej Duda, and US Vice-President Mike Pence also delivered speeches in front of crowds and heads of state on Pilsudski Square.
Mr Duda denounced Nazi Germany's attack as "an act of barbarity", while Mr Pence praised Polish people, saying they "never lost hope".
Earlier on Sunday, two other commemorative events were held in the cities of Wielun and Gdansk, where the first German attacks of the war happened.
"Wielun was to show what kind of war it would be, that it would be a total war, a war without rules, a destructive war," Mr Duda said.
Later on Sunday, Mr Duda appeared to criticise Russia's annexation of Crimea from Ukraine in 2014, alluding to a recent "return of imperialist tendencies" in Europe.
In a warning to European leaders, Mr Duda said "turning a blind eye" to this amounts to "consent to further attacks".
Poland suffered some of the worst losses of World War Two - about six million of its citizens were killed, up to half of them Jews.
Eighty years on, Poland is still demanding compensation from Germany for the death and destruction inflicted.
Recent calls for reparations by Polish Prime Minister Mateusz Morawiecki, whose governing Law and Justice (PiS) party has been accused of fanning nationalist sentiment, have fuelled tensions with Germany.
A Polish parliamentary committee is still assessing the amount of compensation, but Germany argues the matter is settled.
BBC
