The 30-year-old actor told The Hollywood Reporter that he will appear in "a lot" of the show's fourth season.
Finn decided to enlist in the army in the musical drama's third-season finale, but Monteith hinted that the character's fate may not be set.
"Finn would make a good soldier but I think he's still got a lot to think about - he still has a lot going for him outside of the army," explained Monteith.
"His heart is still with Rachel (Lea Michele) and I think he has a lot of reasons to not be in the army as well. I think he'd be torn as a soldier."
He added that Finn and Rachel's romance is part of the show's "long-term trajectory", encouraging fans to "root for the couple to reunite".
"I hope when we look back at it, they will be a legendary couple in television," he admitted.
Glee will return to Fox on Thursday, September 13. The series airs on Sky1 in the UK.