Letheren had been on-loan at Doncaster but failed to break into Sean O'Driscoll's side at the Keepmoat Stadium.
The Welsh under-21 international is now set to train with Mike Newell's side who have been looking to bolster their goalkeeping options.
The Mariners surprisingly released Phil Barnes last month leaving Newell with just on-loan Wayne Henderson and former Leeds youngster Jonathan Lund to choose from.
Letheren started his career with Swansea before moving to Oakwell, but he has been unable to break into the first-team making just one appearance with manager Simon Davey preferring Heinz Muller and Luke Steele.
His solitary appearance came in Barnsley's memorable FA Cup run last season, replacing Muller at half-time in the Third Round tie with Blackpool.