The keeper penned a deal with the Pars after agreeing an extension to his Jags contract that will keep him at Firhill for another two years.
Jim Jefferies' side will be in the hunt for the League One title next term and keeper Scully said: "Thistle gave me a two-year contract so it gives me a bit of confidence that they see me as one for the future.
"All I can do is go out and hopefully have another good season, get more games under my belt and more experience. I was always going to be going back out on loan and Dunfermline was my first option.
"We have unfinished business with not getting promoted last season.
"This year we are the favourites and hopefully we can do the business."