The 25-year-old has signed a two-year contract with the Merseysiders and will make the move at the end of the current campaign.
Saints coach Nathan Brown said: "Richard showed a lot of potential coming through the system at Hull KR and will be a real asset to us next season.
"He possesses a lot of size and will bring some real go-forward to our pack.
"At 25 he is still developing too and we are looking forward to seeing that at Langtree Park."
- Sports & Recreation