We last saw Marsters's character, Dr. Milton Fine (aka Brainiac), in Smallville's fifth season finale.
While it's unclear exactly how James is returning, I can tell you he's reprising the exact same character, he's returning to set very soon and his episodes begin airing in January. He also appears on CBS' Without a Trace this season.
The new season of Smallville premieres on Thursday night.
Dont't forget to also check out: Smallville