The London side's worst run of form in four decades has seen them drop to 10th in the Premier League, 10 points below the top four and just 5 above the drop zone, reports Efe news. Arsenal have 19 points from 15 matches, the same as newly promoted Sheffield United who are 9th thanks to a better goal difference and Newcastle United, sitting 11th.
It was a disappointing debut on Thursday evening at the Emirates for interim coach Freddie Ljungberg, who stepped in after Gunners management fired Unai Emery.
It was a dismal first half for Arsenal, who found themselves backed up against their own goal much of the time. The closest the hosts came to scoring in the opening period was an Alexandre Lacazette header that Brighton goalkeeper Matt Ryan stopped on the line.
Home keeper Bernd Leno was tested several times before the Seagulls took the lead in the 36th minute on a goal by Adam Webster, capitalising on the Gunners' failure to defend a corner.
Neal Maupay nearly doubled the advantage for Brighton before Arsenal were booed off the pitch at the break.
The Gunners started the second half in a higher gear and pulled level in the 50th minute, when Lacazette got his head to a Mesut Ozil corner.
Chances followed at both ends of the pitch. Maupay forced a stop from Leno before Arsenal appeared to surge ahead with a goal by David Luiz off Ozil's free kick.
But the goal was disallowed after a VAR review showed that Luiz was offside. With 10 minutes left in regulation, Aaron Mooy delivered an exquisite cross to Maupay, who converted to put Brighton up 2-1.
Gabriel Martinelli had a chance to salvage a point for Arsenal with a fine header in the 86th minute, but Ryan handled the shot.
Brighton, with their first ever victory against Arsenal, climbed to 13th in the Premier League with 18 points.
In Thursday's other match, Newcastle United defeated Sheffield United 2-0 on goals by Allan Saint-Maxim and Jonjo Shelvey. Sheffield saw their unbeaten streak end after seven matches.
( With inputs from IANS )