The 24-year-old, who won three caps for England in 2009, played under current Cowboys assistant coach Terry Matterson while on loan to Castleford in 2008.
Moore made 138 appearances in Super League after bursting onto the scene at the age of 16 with Saints and was a member of the 2009 Dream Team during a season-long loan spell at Huddersfield. "Our coaching staff has identified Scott as a player who is very capable of doing well in the NRL," Cowboys football manager Peter Parr said.
"He sees joining the Cowboys as the chance to take his game to even higher levels and we'll do everything we can to ensure that happens.
"We're sure he's going to fit in well here and challenge for a place in our 17 for round one."
Moore, who also played for Widnes before returning for a second spell at Huddersfield in 2012, will become only the second Briton to play with the Cowboys after Welshman Jonathan Davies made nine appearances for them in their inaugural season in 1995.
Moore is the second Super League player to join the Cowboys for 2013 after Catalan Dragons full-back Clint Greenshields agreed a 12-month deal.