A versatile attacker, Müller has put pen to paper on a contract until 2020 at the Commerzbank Arena, with the option of a one-year extension.
The twice-capped Germany international spent five years in the Eagles' youth system – but never made a first-team appearance – before joining SpVgg Greuther Fürth as a teenager. Müller, 30, has since also turned out for SV Sandhausen, 1. FSV Mainz 05 and Hamburg, where he was the club's top Bundesliga goalscorer in 2015/16 and 2016/17.
A boyhood Frankfurt fan
"It's a great day for me because I've always remained an Eintracht Frankfurt fan," Müller said. "There's been real progress here in the last few years and I want to contribute to further success. I can hardly wait to get out there and play for Eintracht again."
"Nicolai is a seasoned Bundesliga professional who fits in perfectly here," said Eintracht board member Fredi Bobic. "He's quick and capable of playing in several forward positions. He's a really good character and his injury is now behind him, too."
Müller only featured twice for HSV last season after suffering knee ligament damage on the opening day. In total, the attacking midfielder has made 164 Bundesliga outings for Mainz and Hamburg, scoring 37 goals.
