Janelt has penned a new agreement to 2030.
The midfielder joined in 2020 from VfL Bochum and has developed into a first-choice for the Bees.
Janelt recalled to the club's website: "I came here and played a decent role to develop the club on and off the pitch, and now here we are.
"Four-and-a-half seasons in the Premier League, over 150 Premier League games - I think that says a lot and I’m very proud."
On why he has settled so well at Brentford, the 27 year-old also said: "For me it’s the whole package – it’s the fans, it’s the city, it’s the club.
"The Gtech is our fortress. Everyone knows how difficult it is to play against us, especially when we have the fans behind us.
"This season, we’ve lost just one game so that means a lot for everyone. Half of the credit goes to the fans because without them, we wouldn’t really perform like that.
"I’ve been with some clubs and I know a lot of players who play in different cities and in different countries and honestly, you can mention maybe five clubs where you have such a nice group, such nice people to take care of you. This is very rare in football, and I think that’s why we’ve had success as well."