The 25-year-old Boston man had checked into his hotel in the afternoon before going to see a match between Poland and Costa Rica but could not remember his hotel's name, its address or anything else about it, police spokesman Holger Jureczko said.
"He came into the police station at 3 a.m. and asked for help," Jureczko said.
"The only thing he could remember was paying 10 euros (7 pounds) for a taxi ride to the city centre and that he went past a park and a Mercedes dealer. There are a lot of Mercedes dealers in Hanover but we were able to find the one in the vicinity of a park."
Police took the American to the area that matched his vague description in the city of 500,000 and spent an hour driving up and down streets in that quarter until he recognised his hotel just before dawn on Wednesday.