The Bundesliga side will have the option to buy Younes, 27, when his loan term ends in the summer of 2022.
— Eintracht Frankfurt (@Eintracht) October 3, 2020
Younes, a five-time Germany international, returns to his homeland after half a decade away.
He left Borussia Monchengladbach in 2015 to join Ajax for €2.5m and made a great impression at the Dutch side.
In July 2018, he moved to Napoli as a free agent and has since made a total of 27 appearances.
After spending much of last season on the Napoli bench, Younes has joined Eintracht in search of more game time.
Younes has made five appearances for Germany, all in 2017, scoring two goals.