The Singleton Greyhounds junior, who was already signed until the end of 2018, made his NRL debut in Round 7 in 2016 and has played 27 matches for the Club.
The 23-year-old said he was extremely proud to extend his tenure at the Knights.
“I’m over the moon,” smiled King.
“I’m really happy at the Club, Newcastle is my hometown, it’s where my family is and it’s where my heart is, so I’m really happy to get an extension.
“It’s great to see that the Club has got me in their future plans for the next few years, and I’m excited for what’s to come.
“Every week I go out there and do my best and hopefully come up with some good results, so I’ll keep on doing that and training hard.”
General Manager Football Darren Mooney said it’s great to be able to reward Josh with a contract extension.
“Josh is an impressive young footballer on and off the field and we look forward to his continued development at the Club,” Mooney said.
“He has a thirst for knowledge, has an excellent work ethic, and is always working with the coaching staff to improve his game.