The terms of the deal have not been confirmed but the 24-year-old moves to the Robins on a permanent deal following his Exeter City release at the end of last season.
Ward moved to Fratton Park on loan in January after Alex Bass suffered a broken leg in training.
And despite hoping for game-time after falling down the pecking order at the Grecians, the former Reading youngster didn't make a single appearance for the Blues.
Ward was firmly behind Craig MacGillivray in the pecking order and was confined to just one outing for the reserves in a 1-0 win over Millwall.
His arrival at the County Ground represents the eighth player with Pompey roots to join the Robins in just over two years.