The 39-year-old has signed a contract with the Toffees until January after being released by Wolves in the summer after two seasons at Molineux.
Hahnemann will battle it out with fellow countryman Tim Howard for the starting spot in the Everton goal alongside Slovak 'keeper Jan Mucha.
Toffees boss David Moyes believes the American’s experience could be vital if called upon to play this season and gives them depth in the position.
He told reporters: “We have only got two [keepers] so I felt we needed someone. He is experienced and will give us that bit of extra cover.
"It is difficult when you only have two senior goalkeepers - it only needs one of them to go down and then you are in trouble," Moyes continued.
"I would have liked to have got somebody before the [transfer] window closed if we could, but there were other priorities at the time.
"It gives me someone who can help in the reserves and if we get an injury to a goalkeeper, he has got good experience of the Premier League and we can use him as back-up."