Watford gave Graham Taylor a belated Premiership win but the three points did not come without a nail-biting finale at Vicarage Road.
It was their first win since September, but a pulsating Premiership relegation clash had more than 18,000 on the edges of their seats until the final whistle.
David Perpetuini and Xavier Gravelaine had given Watford a two-goal lead, but goals inside a minute from Luis Boa Morte and a rejuvenated Kevin Davies equalised after the break.
It seemed Dave Jones' side might go on to win, but Taylor's fighting Watford, who have found goals hard to come by in the Premiership, carved out a third goal in the 66th minute.
Gravelaine pounced for his second of the game when a defensive clearance fell to him on the edge of the box. Alec Chamberlain had to tip a Boa Morte shot wide in the closing minutes and Micah Hyde cleared a Dean Richards header off the line.
Watford: Chamberlain, Cox, Palmer, Page, Robinson, Gudmundsson (Gibbs 84), Hyde, Johnson, Perpetuini, Ngonge (Foley 67), Gravelaine (Smith 78). Subs not used: Day, Miller. Booked: Perpetuini, Gravelaine, Robinson, Smith, Hyde. Goals: Perpetuini 17, Gravelaine 31, 65.
Southampton: Jones, Tessem, Lundekvam, Richards, Benali, Boa Morte, Soltvedt, Hughes (Beattie 76), Dodd, Davies, Le Tissier (Ripley 43), Ripley (Kachloul 82). Subs not used: Moss, Bridge. Booked: Davies, Hughes, Richards. Goals: Boa Morte 61, Davies 63.
Referee: M Riley (Leeds).