The 28-year-old could make his debut for the Lions 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.
Meanwhile, Millwall have rearranged their Championship home game against Crystal Palace for Tuesday April 30 (7.45pm).
The match was originally scheduled to be played on Saturday April 13 but the Lions will be playing Wigan in their FA Cup semi-final that weekend.