Pompey Women were beaten in the latest south coast derby, despite enjoying a late flurry of chances.
Sophia Pharoah and Lucia Kendall struck either side of the interval to seal the points – and the National League South title – for Southampton.
Jay Sadler made one change from the side that secured an impressive win at Bridgwater at the weekend.
Emma Jones was restored to the starting line-up at St Mary’s, with Hayley Bridge dropping to the bench.
It was the home side who had the first chance to break the deadlock and Hannah Haughton came tearing from her line to deny Ella Pusey.
Kendall then tried to bend a shot into the far corner of the net, only to see the ball flash narrowly past the post.
There were not many clear-cut chances at either end, although a looping free-kick from Jones was nodded wide by Sammy Quayle.
But Southampton broke the deadlock just before the break, courtesy of a fine long-range effort from Pharoah that flew in off the post.
Half Time: Southampton 1 Pompey 0
It got worse for the visitors just a couple of minutes after the restart, as Kendall finished at the second attempt.
The in-form Haughton then stopped them falling further behind by making a couple of superb saves in quick succession.
First she kept out a close-range header from Leeta Rutherford and then clawed away a Pharoah effort.
Sadler looked to the bench to find a route back into the contest and Dani Lane soon created a chance for Jones, whose shot was held by Kayla Rendell.
The same player had another attempt in the closing stages, courtesy of a half-volley from the edge of the box that struck the post.
And Lane saw a long-range strike tipped over the bar by Rendell, as the Blues’ late pressure ultimately counted for nothing.
Pompey: Haughton (c); Quayle (Rowe 56), Tonks, Gane, McDonnell; Rolf (Bridge 89), Hill (Lane 56), Rowbotham (F.Jones 82); Khassal, E.Jones, Albuery
Sub not used: Walters