Despite a strong start from the Saints during which Rickie Lambert and Guly Do Prado tested Owain Fon Williams from long range, the first half belonged to the hosts.
Gary Jones came within a whisker of his 20th goal of the season with a cleverly worked free-kick before Chris O'Grady had a gilt-edged chance to end his 12-game run without a goal. Matt Done sent him one-on-one against Kelvin Davis, but the Southampton goalkeeper won the battle of wits with a smart save.
But the hosts were ahead in the 28th minute when Nicky Adams' back-post cross found Joe Thompson and he sent a looping header past the flat-footed Davis and into the far corner of the net.
The second came four minutes before the break, O'Grady atoning for his earlier miss when he collected Done's pass and smashed a low drive beneath the outstretched Davies.
Saints had chances after the interval, Jonathan Forte and Radhi Jaidi foiled by Fon Williams while Dean Hammond flicked wide from eight yards after ghosting in behind the Dale defence. Adam Lallana's downward header looked destined for the corner of the net until Fon Williams dived to his right to save and the Southampton midfielder's frustration was compounded when his deflected shot struck the foot of the post.