Full-time: Middlesbrough 0 Brighton 1
MIDDLESBROUGH misfired at the Riverside once again as Matthew Upson's 86th-minute header condemned them to their eighth defeat of the campaign.
Upson headed home Craig Conway's corner in the closing stages to ensure another disappointing afternoon for Aitor Karanka, who has still only recorded one victory since replacing Tony Mowbray.
The Teessiders were the better side for most of the second half, and came close when Lukas Jutkiewicz headed over the bar.
However, Brighton had dominated before the break and might well have gone on to win more comfortably had Ashley Barnes not planted an early header against the post when he was left completely unmarked from a corner.
The defeat leaves Boro just three points clear of the relegation zone in 19th position, with the play-off places fast disappearing out of reach as the festive programme draws near.
Karanka made three changes to the side that drew against Birmingham last weekend, with George Friend, Jacob Butterfield and Jutkiewicz returning to replace Ben Gibson, Marvin Emnes and Kei Kamara.
With Kamara likely to be sidelined for the whole of the festive period, Boro desperately need Jutkiewicz to find some form, but the striker initially struggled to make any impression against a well-organised Brighton rearguard.
While Oscar Garcia might be the manager at the Amex Stadium now, the current Seagulls side was crafted by Sunderland boss Gustavo Poyet during his time on the south coast, and they are clearly keen to maintain the passing principles established by the Uruguayan.
Brighton's players knocked the ball around nicely for the majority of the game, with left midfielder Conway causing particular problems whenever he got the chance to run at Rhys Williams.
Conway's tenth-minute corner set up the first chance of the game, and Brighton should really have taken the lead as Ashley Barnes was afforded far too much room in the area.
Barnes was completely unmarked as he met the ball the yards out, but his header crashed against the base of the left-hand post.
Conway was involved again from open play eight minutes later, beating Williams to cross from the left, and Shay Given was forced to get down sharply to keep out David Lopez's low strike.
Boro's first effort at goal came midway through the opening period, with Mustapha Carayol cutting in from the left in order to drill in a fierce low strike that Brighton goalkeeper Peter Brezovan turned around the post.
Jacob Butterfield also tested Brezovan with a long-range strike four minutes before the break, but Boro lacked creativity in the central areas and were unable to get either Carayol or Albert Adomah into the game for the majority of the first half.
Jutkiewicz was finally furnished with a chance on the stroke of half-time, but summing up a lacklustre opening 45 minutes, the striker misjudged his attempted header from Friend's left-wing cross and looped a tame effort into Brezovan's hands off his shoulder.
That represented a poor end to the opening period for Boro, and things got worse during the interval as Jonathan Woodgate was unable to reappear for the second half.
He was replaced by Ben Gibson, and the hosts' remodelled back four was immediately tested as Stephen Ward fired in a rising drive that Given tipped over the crossbar.
Boro enjoyed their best spell of the game at the start of the second half though, and Brezovan could only parry a smart effort from Carayol, who wriggled free of his marker after Brighton failed to clear a corner. Fortunately, for the visitors, the rebound eluded every Boro player in the box.
The hosts continued to dominate though, and Jutkiewicz out-jumped Adam El-Abd to reach Adomah's cross, but was unable to direct his header beneath the crossbar.
Brighton's second-half attacking was much less effective than their efforts before the break, but the visitors were still able to come up with a sucker punch with four minutes left.
Conway swung over another dangerous delivery from the left, and former England international Upson powered to the edge of the six-yard box before planting a downward header beyond Given.
Middlesbrough (4-2-3-1): Given; R Williams (L Williams 89), Woodgate (Gibson 46), Ayala, Friend; Leadbitter, Whitehead; Adomah, Butterfield, Carayol; Jutkiewicz (Main 72).
Subs (not used): Konstantopoulos (gk), Richardson, Smallwood, Haroun.
Brighton (4-3-3): Brezovan; Bruno, El-Abd, Upson, Ward (Calderon 75); Ince, Bridcutt, Crofts; Lopez (LuaLua 59), Barnes (Ulloa 82), Conway.
Subs (not used): Walton (gk), Dunk, Andrews, Agustien.