The 21-year-old forward made his debut for the Saints in 2012, and has since made 29 Saints appearances, plus 20 on dual registration at other clubs.
Saints coach Nathan Brown believes that theplayer has a bright future at Langtree Park.
“This is great news for both the club and Joe,” said Brown.
“He is a one of a host of younger players who have had a reasonable amount of experience over the past few years and have done well.
“He has a bright future and the more experience he gains and the more he keeps on improving in key areas then the closer he will be to becoming a permanent fixture in the team.
“He has really good natural ability and has made significant improvements from where he was last year.
“More importantly, he is a nice young kid and that is as important as being a good player.”
Greenwood has come through the ranks at the Saints academy, after playing his junior rugby at Saddleworth Rangers.
“I’m really glad to be re-signing at a big club like St Helens,” said the player.
“It is a great opportunity for me to continue my development and continue my dream of being a professional rugby league player.
“I would like to thank Nathan Brown for giving me first team experience as a young player and showing faith in me as he has done with all the young players coming through the system.
“I am going to keep training and working hard to cement my position in the squad.”