The deal is set to run for two years, but also has an option for a third, with the 28-year-old set to make the move ahead of the 2022 season. He could remain at St Helens until the end of 2024, should the option be activated.
Sironen has spent much of this season out through injury, but has been a crucial part of the Sea Eagles' team over the past two seasons, playing 40 games across the 2019 and 2020 seasons.
He debuted for the Tigers back in 2012 and was touted as one of the game's brightest young prospects before shifting to Manly in 2017. He has been prone to injury, but has represented New South Wales City Origin on four occasions.
The hard-nosed back rower scored three tries, made five line breaks and 26 offloads in his 18 games last year, while he had seven line breaks and 46 offloads in 2019.
His injuries this year though have opened the door for other youngsters to join the side at Manly, with Josh Schuster and Haumole Olokau'atu establishing themselves in the first grade second row.
Sironen should walk straight into the St Helens team, where he will replace retiree Joel Thompson. Sironen said he was excited about the move.
"I am very excited about the next chapter in my career and I couldn't have hoped to have signed with a better club," Sironen said.
"Every Australian player knows that the Saints set the benchmark in the English Super League. I felt extremely comfortable from the get-go when speaking to [head coach] Kristian Woolf and [chief executive] Mike Rush and I even had an opportunity to chat to [assistant coach] Paul Wellens about the move.
"It was a very easy decision for me because I always wanted to play in England at some stage and I feel now is the right time to be making the move. My ex-Manly teammate Joel Thompson also told me some wonderful things about the club and its supporters.
"Obviously my focus remains on the Manly Sea Eagles and hopefully I can finish the season on a big note."