Off contract after a one-season stint with St George Illawarra this year, Klemmer had always looked likely to leave the Dragons as they prioritised young talent.
The 31-year-old initially agreed to play for Lakes United in the Newcastle competition, but always had the ability to take up a professional deal if it became available.
"When the opportunity came up, I grabbed it with both hands," Klemmer said of the England move.
"Saints has got such a rich history, and I know they demand success, such is the pedigree that the club has. I can't wait to be part of that environment."
The best young prop in the game when he debuted for Canterbury in 2013, Klemmer played for Australia the following year as a 21-year-old front-rower.
He played 14 State of Origins for NSW and won a World Cup in 2017, and still provided good value for the Dragons this year as he reached 260 NRL games.
"He's coming over with a lot to play for, and speaking to him, it's clear to see how competitive he is," St Helens coach Paul Rowley said.
"We're adding a really good bloke to our group too, and I've been speaking to people who know him well, and they all speak very highly of him."
Meanwhile, Dolphins forward Ryan Jackson has also signed to head to the Super League next year, on a two-season deal with the newly-promoted York franchise.
The 21-year-old played 48 Queensland Cup games, and impressed representing the Maroons in under-19s State of Origin.
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