The 34-year-old, who has played 19 times for Paolo Di Canio's men since joining on loan on transfer deadline day in August, has signed an 18-month deal with the Robins.
In December Di Canio said Ward and left-back Jay McEveley were the star performers for his promotion-chasing Town side during the first half of the season and he has made no secret of his desire to sign the former Wolves man on a permanent basis.
Ward began his career at Watford and played for the likes of QPR and Crystal Palace as well as spending two spells with Millwall.
