The 39-year-old Macedonian, who had only recently extended his deal to 2014 with Frankfurt after joining in 1994, had asked to opt out of his contract after receiving several offers from American clubs.
"Because of his long merits and outstanding performances for the club, Frankfurt agreed to grant him his wish after a 20-year commitment and with great regret to release him from his contract," the club said in a statement.
The German-born Nikolov did not specify which clubs were interested in signing him.
"Eintracht is my one and all," said Nikolov. "But throughout my career I always had the dream of playing in America to widen my horizon and I want to fulfil this dream now."