The 55-year-old actor, who played Tallahassee in the 2009 film, says the second installment will not make sense if the full cast doesn't return, reported DigitalSpy.
"I think it'll probably happen. It would have to be the whole gang," Harrelson said.
However, co-writer Paul Wernick hinted that the increased star status of the cast since the first film could be an issue.
The emsemble cast also included Emma Stone, Jesse Eisenberg and Abigail Breslin.
"All the cast have become superstars now. We made 'Zombieland' with USD 20 million, so it's trying to fit that financial model into the sequel model so it makes sense for the studio and being able to pay the actors what they now get paid and deserve to be paid," Wernick said.