Lees has been a one-club player in the English Super League, joining the St Helens academy in 2015 as a 16-year-old before making his club debut in 2017 at age 18.
He has since racked up 199 Super League appearances for the club despite missing most of the 2026 season with a serious knee injury.
Lees was recently replaced by former NRL player Daniel Suluka-Fifita, who signed for the club a week ago after making 46 appearances in the NRL for the Rabbitohs, Roosters and Bulldogs.
PNG Chiefs General Manager Football Michael Chammas hailed Lees as "one of the toughest and most respected forwards in rugby league" when announcing the signing on Tuesday.
"He has built his career through hard work, consistency and an uncompromising approach to the game," Chammas added.
"Those qualities are exactly what we're looking for as we build our roster and culture for 2028.
"We're delighted to secure Matty as Player 004 and look forward to seeing the impact he'll have both on and off the field."
Lees follows the existing signings of Jarome Luai, Alex Johnston and Connor Watson, the latter of whom also confirmed his moves on Tuesday.