The French side signed Trapp from Eintracht in 2015 and he won a string of trophies during his time with the capital club, including two Ligue 1 titles, two Coupes de France, two Coupes de la Ligue and four Trophees des Champions.
Now, he'll return to Frankfurt, where he featured from 2012-15 after originally breaking through with FC Kaiserslautern.
"I made it clear in the spring that I feel very comfortable in Frankfurt with Eintracht," said Trapp.
"The time in Paris was very exciting, successful and a great experience for me. Now I'm coming home completely and I want to achieve a lot here."
Trapp made 96 appearances in his first spell with Frankfurt across his three initial seasons and featured 45 times in all competitions during his season on loan.
The goalkeeper has won three international caps for Germany, having been a part of the successful run at the 2017 Confederations Cup as well as Germany's 2018 World Cup squad in Russia.
"Kevin was an important pillar last season and played a huge part in the positive season we had," said Eintracht sporting director Fredi Bobic.
“He is enormously important to the team and we are very happy to be able to sign a top goalkeeper and German national player for the long term."
Frankfurt are set to face FC Vaduz on Thursday in the third round of Europa League qualifying, having topped Estonian side Flora in the prior round.