Leandro Trossard, an Arsenal substitute, scored his team's first goal, but the tenacious Brentford team earned a well-earned point thanks to Ivan Toney, preventing the Premier League leaders from extending their lead to eight points.
After a scoreless first half during which the Bees' Toney struck the post, the Gunners took the lead four minutes after Trossard came off the bench.
But Toney's close-range head nod of Christian Norgaard's cross gave Brentford the just reward for their outstanding away performance at Emirates Stadium.
The goal was eventually confirmed after a protracted investigation by the video assistant referee for a potential offside in the build-up.
Brentford finished off an excellent all-around defensive performance by holding on for a point as Arsenal stepped up the tempo in the final minutes, including seven minutes of added time.
On February 15, Arsenal will square off against Manchester City, who had fallen six points behind before their game against Aston Villa on Sunday.
While this is going on, Brentford is eighth and only five points behind fifth place.
Gunners' title hopes are damaged by the tenacious Bees.
Before Brentford's visit, Arsenal had an impressive home record, winning eight and drawing one of their nine games at the Emirates.
Along with Newcastle, they are one of only two teams in the top flight who have never lost at home. However, this record was put to the test against an in-form Brentford team who had won four of their previous five league games.
Although Mikel Arteta's team had a lot of possession, they struggled to break down the Bees' well-organized defense. Their perseverance was rewarded when Trossard scored midway through the second half.
Since joining from Brighton and Hove Albion for £27 million last month, the Belgian forward has seemed like a fine addition, and he came within inches of adding a second with a long-range shot shortly after.
But it never seemed like a one-goal advantage would be enough against a Brentford team that had been aggressive in attack, as Toney's equalizer showed.
The draw comes after consecutive losses to Everton and Manchester City in the league and FA Cup, respectively.
It will be crucial in determining whether they can win a first Premier League championship since 2003–04 to see how they react to this temporary setback, not least in Wednesday's home game against City.
Bees in good shape are looking toward Europe.
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 season last year.
Before traveling to Arsenal, Thomas Franks' team had not given up a goal in any of their three league games, and it was easy to see why they are such a hard team to beat.
Despite having the majority of the possession, they maintained their composure and brilliantly adjusted to the Gunners taking the lead to become one of just two teams this season to leave anything at the Emirates.
Only Erling Haaland and Harry Kane have scored more goals this season than Toney's 15, and the Bees have a strong defense to go along with his threat, so they have a good chance of qualifying for Europe next season.