The 48-year-old Dane has been in charge of the Bees since October 2018, having initially joined as an assistant coach two years earlier.
Frank guided Brentford to the Premier League for the first time at the end of the 2020/2021 season via the championship play-offs.
Brentford have enjoyed a strong start to their maiden Premier League campaign, sitting in 14th position and eight points clear of the relegation zone.
Frank’s assistant Brien Riemer has also extended his contract until 2025.
Brentford director of football, Phil Giles, said: "On behalf of everyone at Brentford, I'd like to congratulate Thomas and Brian on their new contracts.
"First and foremost, they are fantastic people to work with and I'm sure everyone at the club will be pleased that they have committed to stay with us until 2025.
"They have been integral to the success that we've shared over recent seasons, culminating in achieving our shared target of playing in the Premier League.
"We want to continue to take Brentford forward, to compete with clubs far bigger than us, and to see how far we can progress.
"I am sure that we can build on the success that Thomas and Brian have helped deliver along with all of the other staff and players and look forward to what I hope will be a successful conclusion to the season and beyond."
