Lillis, 24, has penned a two-year contract at Spotland, returning to a club where he has already had two spells on loan.
Lillis began last season as the Iron's first-choice keeper following the departure of Joe Murphy, but suffered a knee injury early in the campaign and was unable to dislodge Sam Slocombe on his return.
He came through the ranks at Glanford Park, making a total of 50 first-team appearances.