Durm's contract at Huddersfield ended after the Terriers were relegated from the Premier League and he has returned to his homeland, where he previously played for Borussia Dortmund.
The full-back, who can play on either flank, made 28 Premier League appearances for Huddersfield in 2018-19 as he bounced back from an injury-ravaged final season at Dortmund.
Huddersfield had an option to extend Durm's contract by a further 12 months yet decided against retaining his services, though the 27-year-old enjoyed his time in England.
"My year in England was a great experience," said Durm.
"The Premier League is one of the best leagues in the world, with many great teams and individual players.
"Even though we did not manage to maintain the class in the end, I draw a lot of positives from this time."
On his new club, he added: "When the offer came from Frankfurt, I was very positive from the beginning.
"Because my impression of the club was great, when the talks were held I quickly realised that I wanted to take this step."
Eintracht continued a busy day by bringing in Dominik Kohr from Bayer Leverkusen.
The 25-year-old midfielder has joined on a five-year deal for a reported €10 million fee.
They did, however, lose the services of star striker Luka Jovic, who completed a €70m (£62m/$79m) move to Real Madrid in the summer transfer window.
Frankfurt face Hoffenheim in their first match of the 2019-20 Bundesliga season, with the game due to take place on either August 17 or 18.