The Swedish international has moved to Griffin Park in a £5.5m deal following a breakdown in relations at Elland Road.
Jansson had been training with former club Malmo after being granted extended leave by Leeds. He flew out yesterday to join the rest of the Brentford squad at a pre-season training camp in Austria.
Before doing so, the 28-year-old issued a statement via social media that included a big thank-you to both United’s supporters and his former team-mates.
“You took me into your hearts and have been fantastic since day one,” he wrote. “Biggest of thanks to my team-mates that have supported me and been by my side since the first session.
“As I loved to play for the club, Leeds will always be part of me and my family.”
Jansson also went on to express his happiness at joining Brentford.
“I really believe in this club and this squad and cannot wait to get going,” added the defender. “A new adventure and a new step so I am looking forward to it.”
As Jansson flew out to join up with his new team-mates, Leeds were continuing their preparations for tonight’s friendly at York City.
It is the first outing of pre-season for both clubs, United also making the short trip to Guiseley tomorrow evening before then flying out to Australia.
A bumper crowd of more than 4,000 is expected as supporters take the opportunity to watch any summer signings.
Helder Costa and Ben White are the two new faces at Elland Road, Marcelo Bielsa’s two other additions being familiar figures in Jack Clarke and Jack Harrison.
York manager Steve Watson, meanwhile, has been busy as he tries to plot an escape from the sixth tier. Steve McNulty, the vastly experienced former Tranmere and Luton defender, is the latest addition to a squad that has also welcomed Andy Bond, Kieran Green, Peter Jameson and Dan Maguire.
Jay Rodriguez has rejoined Burnley from West Brom, the Premier League club have announced. The 29-year-old striker has signed a two-year deal to return to his hometown side, with the option of a further year, for an undisclosed fee.