Brentford s Rico Henry, right, duels for the ball with Manchester City s Riyad Mahrez, second right, and Manchester City s Kyle Walker during the English Premier League soccer match between Brentford and Manchester City at the Gtech Community Stadium in London, Sunday, May 28, 2023. AP
Flekken agreed a four-year contract with the Premier League club, who will complete the transfer subject to international clearance and a work permit being granted.
The 29-year-old, capped four times by the Netherlands, could replace Brentford's current first choice keeper David Raya, who has been linked with a close-season move.
"We're very happy to have signed Mark. He's a player with a lot of quality who will strengthen our goalkeeper group," Brentford manager Thomas Frank said.
"He has big experience and joins us after two great seasons with Freiburg, who have just had a top season in the Bundesliga, finishing fifth and qualifying for the Europa League.
"He was a big part of the team there and now he's coming to us and will hopefully make the group better and the team better.
"He's a very good shot-stopper and comfortable with the ball at his feet. His distribution is very impressive."
Flekken began his career in his native Netherlands at RKVV WDZ and Roda before moving to Germany, where he has played for Alemannia Aachen, Greuther Fuerth, Duisburg and Freiburg.
He made his Netherlands debut in March 2022 and kept 13 clean-sheets in 34 Bundesliga matches, the highest tally in the division last season.
Frank has said Raya could leave the club after four successful seasons.
But Frank warned clubs that it will cost £40 million to sign the highly-rated 27-year-old.