The Swedish international has signed a three-year deal with the Bees, who also have an option to extend the defender’s stay by a further 12 months.
A breakdown in relations is understood to be behind the surprise move for Jansson, who made 120 appearances for the Elland Road club after joining from Torino.
Jansson had been training with former club Malmo after being granted extended leave by Leeds. He will fly out today to join the rest of the Brentford squad at a pre-season training camp in Austria, while Leeds continue their own preparations for tomorrow’s friendly at York City.
It remains to be seen whether United bring in a replacement for Jansson, who made 39 appearances last term as Marcelo Bielsa’s side finished third in the Championship.
Ben White, a 21-year-old defender, moved to Elland Road last week on a season-long loan from Brighton & Hove Albion but he is yet to play a game in the top two divisions of English football.
On the capture of Jansson, Brentford’s joint director of football, Phil Giles, said: “Pontus is not a player we have been tracking for some time. In fact, it was only very recently that we were alerted to his availability and reacted quickly to the opportunity.
“It says a lot about Pontus that he chose to sign for Brentford as much as we chose to sign him. There is a mutual feeling that we can be successful together, and we wish him well during his time with us.”
Thomas Frank, Brentford’s head coach, added: “We are very pleased with Pontus’s arrival. The defence is an area that we have been looking to strengthen and we feel that Pontus will bring a lot to the group. Pontus will add to the quality and competition for places that we need to adapt to different situations over the course of the season.
“Pontus is a top defender who brings immense professionalism, a great attitude, experience, defensive robustness, leadership qualities, and threat from set pieces to our side.”