Wigan have announced Uwe Rosler as their new manager after agreeing compensation with Brentford.
Marseille have parted company with manager Elie Baup in the wake of Friday's defeat to Nantes at the Stade Velodrome.
The 27-year-old is reunited with Royals boss Nigel Adkins, who managed him at St Mary's and also at Scunthorpe.
Sharp has scored 140 career goals but has not made an appearance for Southampton since August 2012, his last action coming in a loan spell with Nottingham Forest last season.
Nevertheless he will go straight into the Reading squad for Saturday's visit of Birmingham.