The 18-year-old fullback, who played two NRL matches during the 2016 finals series, was a member of the 2016 NYC Team of the Year and is rated one of the most promising young players in rugby league.
"There is no doubt Kalyn is a very talented and exciting young player and his signing is definitely a coup for the club," Knights coach Nathan Brown said.
"We look forward to further developing his talent and welcoming his family to Newcastle."
Knights CEO Matt Gidley said Ponga remained contracted to the Cowboys for the 2017 season.
"As it stands, we are excited about Kalyn joining us in 2018," Gidley added.
"Should the Cowboys decide to release Kalyn before the conclusion of his contract, we would certainly welcome him to the club at an earlier date."
