The defender, 26, has signed a two-year deal at Oakwell after the Tykes paid an "undisclosed five-figure" fee to Brighton & Hove Albion for his services.
McNulty was on loan at Glanford Park from July until December last season, but managed just seven appearances as he struggled with an ankle injury which eventually cut short his stay in North Lincolnshire.
Barnsley boss Keith Hill has also secured the signings of Miles Addison from Derby and Rochdale pair Matt Done and Scott Wiseman.