Welham, 22, has been in excellent form this season and will now stay at Craven Park until at least the end of 2012.
He said: “Hull KR is the place to be and this is the place for me.
“This is my hometown team and to play for them for the next three years is going to be very special.”
Welham was first given a chance by coach Justin Morgan in National League 1, and has stepped up to the standard of Super League in the past three years.
He joined Rovers from local amateur club Myton Warriors.