Portsmouth have named Richie Barker as their new manager and Steve Coppell as director of football.
Wigan have announced Uwe Rosler as their new manager after agreeing compensation with Brentford.
The 28-year-old could make his debut in Saturday's Championship derby at The Valley and will be available for the FA Cup semi-final against Wigan next month, as well as the final on May 11 if Millwall defeat the Latics.
Chaplow made three starts in the Capital One Cup for Saints earlier this season but has had to settle for three appearances off the bench in the Premier League, the last of them coming in the 2-2 draw at Chelsea on January 16.
Chaplow started his career at Burnley before moving to West Brom for £1.5m in January 2005. He then joined Preston before signing for Southampton in January 2011.