The 22-year-old, who has played 26 matches for the Salford City Reds this season, heads to the club as part of Rangi Chase's move to the Reds.
"I am really pleased to have signed Marc for next season. He is a quality player with the added benefit of an outstanding left-foot kicking game," Sky Sports News quotes Castleford coach Daryl Powell as saying.
"He will add to the depth and quality of our option within the squad moving into next season."
Sneyd has scored four tries for Salford this season.