Sharp joined Saints in the January transfer window for a fee in the region of £2million and scored two second-half goals to earn victory as the hosts continue to march towards promotion.
It will have been a bittersweet afternoon for Sharp as, despite sending his new side nine points clear of third-placed West Ham, the defeat leaves Rovers, for who he scored 41 goals in 86 appearances, staring relegation in the face, five points adrift of safety.
It was a dominant performance from the hosts and Sharp broke the deadlock shortly before the hour with a header and then put the game to bed by firing home from inside the area with 15 minutes to go.
Rovers were left to reflect on a third successive defeat, which may have been different had Frederic Piquionne headed home early on.
Seven wins out of nine had seen the Saints become firm favourites for the title before kick-off and Sharp was given the nod upfront against his old club.
For Rovers, back-to-back home defeats to Derby and Millwall had damaged their chances of survival but they will have been buoyed by a recent point at West Ham.
Rovers' start will have provided further optimism for boss Dean Saunders, but after complaining of missed chances in recent weeks, he was again left with his head in his hands when Piquionne headed over from Simon Gillett's cross in the eighth minute as Rovers threatened to cause a shock.
The feeling was that the miss could prove costly as Southampton settled in from there and Adam Lallana could have opened the scoring after a neat exchange with Sharp, but his effort was blocked.
The traffic was becoming one way and after Rickie Lambert had a shout for a penalty turned down, Sharp went close to registering against his old side, though his header went wide of the post.
The opener eventually came 13 minutes after the restart when Sharp reacted quickest to get on the end of Dan Fox's inviting corner to head home his second goal in a Southampton shirt.
That tally was soon taken to three with 15 minutes remaining when he capitalised on some poor defending from Habib Beye before firing past Carl Ikeme.
Richard Chaplow shot wide as the Saints looked to put the icing on the cake, but a fourth win on the bounce means it appears as though it is just a matter of time until promotion is secured.