Leandro Trossard, an Arsenal substitute, scored his team's first goal, but Ivan Toney gave tenacious Brentford a well-earned point, preventing the Premier League leaders from moving eight points clear atop the standings.
The Gunners took the lead four minutes after Trossard came off the bench, ending a goalless first half in which the Bees' Toney hit the post with a shot.
Toney's close-range nod of Christian Norgaard's cross, however, gave Brentford the reward their outstanding away performance at Emirates Stadium merited.
After a protracted investigation by the video assistant referee for a potential offside in the build-up, the goal was eventually determined to have been scored.
Brentford completed a superb all-around defensive performance by holding firm for a point as Arsenal increased the tempo in the final minutes, including seven minutes of added time, as they searched for a winner.
On February 15, Arsenal will face Manchester City, who trailed by six points before their match against Aston Villa on Sunday.
Meanwhile, Brentford are eighth and are only five points behind fifth place.
Bees' tenaciousness damages Arsenal's title hopes.
This season, Arsenal has been dominant at home, winning eight and drawing one of their nine games played at the Emirates before the arrival of Brentford.
Their unbeaten home record was put to the test against an in-form Brentford team that had won four of their previous five league games. They are one of only two teams, along with Newcastle, in the top flight to have never lost on their own ground.
The patience of Mikel Arteta's team paid off when Trossard scored midway through the second half despite having a lot of possession but struggling to break down a well-organized Bees defense.
Since joining from Brighton and Hove Albion for £27 million last month, the Belgian forward has appeared to be a valuable addition. He came within inches of scoring again shortly after with a long-range shot.
However, a one-goal advantage didn't seem to be enough to hold off a Brentford team that had been attacking well, as Toney's goal proved.
Arsenal lost to Everton and Manchester City in consecutive league and FA Cup matches before the draw.
The way they react to this minor setback, especially in Wednesday's home game against City, will be crucial in determining whether they can win the Premier League title for the first time since 2003–2004.
Aiming for Europe are the Bees in form.
Brentford is having another strong season and is vying for a spot in the European League after placing a commendable 13th in their inaugural Premier League campaign last year.
Prior to the trip to Arsenal, Thomas Franks' team had not given up a goal in any of their three league games, making it abundantly clear why they are such a challenging team to beat.
They stayed in shape even though Arsenal had the majority of the possession and brilliantly recovered when the Gunners took the lead to become one of just two teams this season to win at the Emirates.
Only Harry Kane and Erling Haaland have scored more goals this season (15 each), and with Toney's threat and the Bees' strong defense, they have a great chance of qualifying for Europe's elite league next year.