Middlesbrough slumped to a 4-3 defeat at Fulham and Norwich could only draw 1-1 at Rotherham, sealing Watford’s return to the top flight after an eight-year absence.
The Hornets, who now boast five successive wins, ground out a workmanlike success before a crowd of 28,841 at the Amex Stadium.
Defeat for Brighton, their fifth in the last six games, followed a familiar theme as they again failed to take their chances despite an improved display.
The Seagulls should have taken something from the game and defender Joe Bennett wasted an opportunity to grab them a point late on, before Czech substitute Matej Vydra sealed the points with a cool finish in the last minute of stoppage time.
Watford, unbeaten in their previous eight away games, were put under early pressure and Brighton had the chance to take a first-minute lead.
A fine ball through the middle by centre-back Lewis Dunk split the visitors’ defence and, after Chris O’Grady had a left-footed effort blocked, Dale Stephens failed to get a shot away when well placed 12 yards from goal.
The Hornets threatened when Troy Deeney, the first Watford player to hit the 20-goal mark in three successive seasons, had a right-footed shot saved by former Fulham goalkeeper David Stockdale.
Brighton’s Championship status was effectively assured for another season following Millwall’s 2-0 defeat at Blackburn on Tuesday.
And, encouragingly for boss Chris Hughton, his men started with confidence and defender Bennett volleyed wide before having a penalty appeal waved away following a clumsy challenge by Juan Carlos Paredes.
Watford head coach Slavisa Jokanovic made a tactical change in the 25th minute with Daniel Tozser replacing fellow midfielder Ikechi Anya.
Four minutes later, skipper Deeney broke the deadlock with his 21st goal of the season with a well-taken right-footed shot from seven yards.
A cross by Nigerian striker Odion Ighalo was only half cleared by defender Greg Halford and the ball fell kindly for Deeney, who gave Stockdale no chance by smashing home an impressive strike with his favoured right foot.
Brighton went into the game having scored only three goals in their previous eight matches and poor finishing again surfaced when O’Grady shot wastefully wide in the 34th minute.
The Hornets’ Algerian midfielder Adlene Guedioura had a cross cleared shortly after the restart before Hungarian substitute Tozser sent a 25-yard free-kick narrowly over the crossbar.
Keeper Stockdale came to Albion’s rescue with an important save to deny Guedioura from a tight angle as the visitors continued to offer a threat.
Seagulls boss Hughton made his first change when midfielder Mustapha Carayol, making his final appearance on loan from Middlesbrough, was withdrawn in favour of departing striker Craig Mackail-Smith.
The Watford defence was put under pressure when, following a corner by Stephens, skipper Gordon Greer got above his marker but directed his header wide at the far post.
Swiss international midfielder Almen Abdi shot wide in quick succession for Watford, who were roared on by more than 3,000 travelling fans.
O’Grady came close to grabbing an equaliser for Brighton six minutes from time when, after a fine delivery from substitute Inigo Calderon, he headed narrowly wide at the near post.
But Deeney slipped in Vydra for his 15th goal of the season to wrap up victory and ramp up the pressure on the promotion-chasing pack.