George Williams has played 330 first-grade games and will join the Dolphins for the 2027 and 2028 seasons.
The club confirmed the signing Tuesday evening while Williams is in Australia for the World Cup.
He was linked with the Raiders early in 2026, and his current club, Warrington, released him mid-year.
Williams will replace Kodi Nikorima, who joins St George Illawarra this off-season, and partner Isaiya Katoa under coach Kristian Woolf in 2027.
Brad Schneider will also provide cover in the halves.
Williams began his career at Wigan in 2013, making 179 appearances before a year-and-a-half with Canberra; he has played for Warrington since.
Dolphins CEO Terry Reader said, "He has performed at the highest level in both the NRL and Super League and has proven himself as a leader on the international stage with England."