
Frank, who took charge in 2018, led the London club to the top flight for the first time in 74 years.
The Dane guided Brentford to a 13th-placed finish in their first season in the Premier League.
“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,” Frank said in a statement.
Frank last signed an extension in January this year, and his previous contract was set to run until 2025.
Brentford, who are 10th in the league, host fourth-placed Tottenham Hotspur on Monday.