The 24-year-old is set to be reunited with former Iron boss Nigel Adkins at St Mary's.
Forte has made more than 100 appearances for Scunthorpe and has found the net four times this season.
His signing could signal the end of Jason Puncheon's time on the south coast with the winger widely expected to leave and a host of clubs, including Millwall, linked with his signature.
Source: PA
Source: PA