22nd September 2017, 12:02 | Castleford
The 23-year-old winger is joining the Betfred Super League leaders on a two-year deal, with an option for a third season, from Kingstone Press Championship club Sheffield Eagles.
Lo, who has scored 45 tries in 44 appearances for the Eagles, will be loaned back to Sheffield when not required by the Tigers, who have also agreed to play a pre-season friendly with the Championship club.
Castleford coach Daryl Powell, who has also signed Newcastle Knights' former Huddersfield Giants centre Joe Wardle for next season, said: "Garry Lo has been a really exciting player for Sheffield over the last two years.
"I've had a good look at him and I think he will fit perfectly into the way that we play. He's an exciting player, he's very robust and a strong ball carrier who's got an ability to pass the ball as well, so I think he'll be a very exciting player to add into what we have already got."
Lo, who played for PNG Hunters in their inaugural season in the Queensland Cup before coming to England, said: "I'm speechless! To be given this opportunity is a dream come true for me.
"To get the chance to test myself at the highest level in Super League is so exciting and I can't wait to join the boys soon for pre-season training."
Lo is expected to play for Papua New Guinea in the end-of-season World Cup.