Szalai joined Fenerbahce in January 2021 from Cypriot side Apollon Limassol. He helped them win the Turkish Cup in the 2022-23 season as well as finish second, behind champions Galatasaray, in the Turkish top flight.
"It is our clear goal to further stabilize our defence and Attila has already proven his qualities internationally," Hoffenheim Managing Director Alexander Rosen told the club website.
Hoffenheim did not disclose the transfer fee but German media reported the deal could be worth 12 million euros.
"I had several options for my sporting future, but I made a conscious decision to go with Hoffenheim. I really wanted to play in the Bundesliga and I'm convinced that it's the right step for me," Szalai said.
Szalai made his Hungary debut in November 2019 in a friendly defeat by Uruguay and so far has made 35 appearances for his country. He was included in their squad for Euro 2020.
