Eastmond was capped four times by England at rugby league during his five seasons with St Helens, then winning six England caps after switching to union with Bath in 2011.
He played for England under head coach Stuart Lancaster and scored one try, but did not make the 2015 World Cup squad.
He later played for both Wasps and Leicester Tigers before returning to the 13-man code to end his career with Leeds Rhinos in 2021.
Since finishing his playing career he has had coaching stints across the two codes, with spells at Rotherham and Jersey Reds in union and most recently at Warrington Wolves in league.
His new club Halifax want to tap into all the experience he has gained across the two sports.
“As a dual-code international with England, Kyle brings with him a winning mentality developed under some of the best in the business,” said Halifax chief executive officer Damian Clayton.