The Tonga international, 30, has signed a one-year deal after spending the last three seasons at Emerald Headingley.
Leeds offered him a new deal but he turned it down in favour of a switch.
He links up once more with Krisitan Woolf - who is coach of St Helens and Tonga - and said: “I am honoured to sign with this team.
“The Saints have won three titles in a row and are currently the best team in Super League.
“I am blessed to have the opportunity because a lot of players and young kids want to play for this team. I am truly grateful.
“I like a challenge and I know it is going to be tough to find a spot in the squad, but it is a challenge I am looking forward to.”
Hurrell lost his starting spot in the Leeds side towards the end of this season but remains one of the game’s most explosive players when on top of his game.
Saints have already signed Canterbury Bulldogs centre and Tonga team-mate Will Hopoate for 2022 to help offset the loss of Kevin Naiqama and also have England centre Mark Percival in their ranks.
“I am looking forward to proving a point that I can be good enough to be in this team,” said the ex-Gold Coast Titans and New Zealand Warriors star.
“I want a big pre-season because I know I need to work hard and earn the respect from the boys.
“The one thing I love about the Super League is the fans and hopefully the St Helens fans are looking forward to seeing me in a St Helens jersey.”
Woolf said: “Everyone knows ‘Koni’ is a great character and he has a terrific ability to enjoy rugby league and make the fans feel a part of the team.
“What I also know is he is a terrific competitor and I know he has not been happy with what he has done in 2021.
“There are a number of reasons for that; people may forget he has had some family tragedy through lockdown which made last year a really tough year for him and that sometimes gets forgotten.
“He is a strong family man and that does affect your footy and what you can do on the field.
“I have been really proud watching him get through that as well as overcome some injuries and I am really happy he is joining us.
“I know what he is as a player and he has played in some real big games for Tonga and been one of our best players.
“The 2017 World Cup beating New Zealand is an example, he really stood up in that game as he did against Great Britain and Australia in 2019.
“He was in the 2019 and 2020 Super League Dream team and coming into our environment next season I think he will really thrive in it. We will get him at his fittest and at his best and if we do that and he is happy, we will have a real player on our hands.
“He can be a real X Factor for us next year and I am sure the fans will love him.”