Welsh-qualified Lonsdale – who can also operate in the back row – will move to Rodney Parade for the start of the 2022/23 season after a six-year spell with English giants Exeter Chiefs.
The 24-year-old – who stands 6ft 4in tall and weighs 17st 6lbs – moves back to Wales after previously spending time with RGC in the Welsh Premiership.
Lonsdale follows JJ Hanrahan, Bradley Roberts and Rhodri Jones as being confirmed as new recruits by the region.
Dean Ryan, Director of Dragons Rugby, said: “We’re really pleased that Sean will be joining us ahead of next season.
“He’s young player who is keen to be a part of the Dragons set-up and take this opportunity to return to Wales and make a big impression with us.
“Sean has learnt a huge amount during his time at Exeter and we look forward to what he can bring to our environment as we continue to build and move forward.”
After spells with Taunton and Plymouth Albion, Lonsdale made his English Premiership debut on the opening day of the 2018/19 season.
The hard-hitting forward has gone on to become an established member of the Chiefs side, scoring five tries in 61 appearances.
Lonsdale has won two English Premiership titles as well as the European Champions Cup.
He said: “I really liked what Dean had to say about the region and I can see it being a great fit for me.
“His vision for where the team is heading is exciting. The region has a huge potential and I’m looking forward to being a part of it.
“It’s going to be a be a big change playing rugby back in Wales and in the United Rugby Championship.
“It’s a very competitive league which will present new challenges, but I’m confident that it will only help me grow and develop as a player.”
Lonsdale also thanked Exeter Chiefs for the club’s support adding: “I am truly thankful to all the staff and players who have helped me develop over the last six years at Exeter.
“It’s been an incredible experience getting to play and train with some of the best coaches and players there are, and I am very grateful to have been in that position.”
