Abdoulaye Doucouré’s late equaliser denied Southampton their first Premier League win since November.
James Ward-Prowse, who has scored three goals in his last two games in all competitions, struck twice, but second-half goals from Andre Gray and Doucouré pegged Southampton back in a 2-2 draw.
Southampton appeared set for their fifth win of the season, but for a second league fixture in a row they dropped points from a winning position.
A timid opening 20 minutes was turned on its head when Ward-Prowse tucked Shane Long’s cut back into the far corner to give Southampton a 1-0 lead.
Marvin Zeegelaar then denied Long a certain goal with a strong block after Dusan Tadic picked out the forward at the far post on 33 minutes.
Watford was void of an true chance on target in the opening half and Christian Kabasele squandered a lone chance as his header from a dangerous position looped over the bar.
Ward-Prowse would make it 2-0 on the stroke of half-time following a superb counter attack led by Long, who had been a thorn in the side of the Watford defence.
Long made a surging run down the left before playing the ball into Tadic who calmly laid the ball off for Ward-Prowse to place his shot into the corner.
Watford started the second-half with promise, but could not bring their pressure to a head.
Gray, however, pulled one back for the Hornets short of the hour mark when he turned in the rebound after Alex McCarthy tipped Daryl Janmaat’s shot off the bar.
Doucouré then grabbed the equaliser for his third goal in three games against Saints in added time as he pounced on a loose ball and was not judged to have used his hand in the build up.
Watford: Gomes (GK), Janmaat, Wagué, Kabasele, Zeegelaar, Doucouré, Watson (Deeney 46), Carrillo, Cleverley (Pereyra 35), Richarlison, Gray (Okaka73).
Unused subs: Karnezis (GK), Prödl, Mariappa, Holebas.
Goals: Gray (58), Doucouré (90).
Southampton: McCarthy, Cédric, Stephens, Hoedt, Bertrand, Højbjerg, Romeu, Ward-Prowse (Sims 92), Davis (Lemina 63), Tadić (Boufal (91), Long.
Unused subs: Forster (GK), Pied, Bednarek, Gabbiadini.
Goals: Ward-Prowse (20, 44).
Bookings: Højbjerg (32), Romeu (79), Tadic (88).