The 49-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 secured a creditable position in their maiden Premier League campaign, finishing 13th and eleven points clear of the relegation zone.
The Bees are currently 10th on the league table and secured a famous 2-1 win away at champions Manchester City just before the league went on a break for the World Cup.
Frank only signed a new contract in January this year but Brentford have moved to extend his stay further.
"Being a head coach at a club is like a relationship; there are highs and lows," Frank told the club's website.
"I have been here six years in total, which is a long time in modern football, and I'd like to say thank you to the fans for the support you give us – the players, the staff, everyone involved with the club.
"The warmth, support and kindness every time I meet a Bees fan has been amazing and it gives me extra energy to continue the work going forward.
"We attacked the Premier League in the first season and we're attacking it in our second season. I'm looking forward to hopefully continuing our progress and creating more magic moments together."
Director of football Phil Giles added: “I’m sure I speak for everyone at Brentford when I say that I am very pleased that we have agreed this new contract with Thomas. It is well deserved following some excellent results and performances in the last 18 months.
“Thomas has a detailed knowledge of the game, but just as importantly is able to inspire the players and build strong relationships with our staff.
“He has proven himself to be a Premier League-level head coach but retains an open-mindedness to keep learning and improving himself.
“The recent addition of Claus Norgaard as an assistant first team coach, along with the opening of the Robert Rowan Performance Centre, gives Thomas, me and all the players and staff new energy to keep working hard and seeing how far we can take the club.”
