Welsh, 19, has been playing amateur rugby league for Upton having previously been part of the Hull scholarship and Castleford ACER programmes.
The prop said: “I’m really grateful to Doncaster for giving me this opportunity and I hope I can push for a place in the seventeen.
“I think we have a great squad of players and there seems to be a good mixture of youth and experience which could prove to be beneficial for us.”
Doncaster face Hunslet and Sheffield in friendlies at the Keepmoat Stadium this month, before getting their Northern Rail Cup campaign underway at Gateshead on February 12th.
Dons coach Tony Miller said: “Fran is yet another young and exciting prospect we have brought to the club and I know he will do his best to secure a place in the pack.
“We have a number of options in the forwards and this can only be good news for the Dons.”