Substitute Leandro Trossard scored his first goal for Arsenal but the Premier League leaders missed the chance to move eight points clear at the top as Ivan Toney earned resilient Brentford a deserved point.
After a goalless first half in which the Bees hit the woodwork through Toney, the Gunners took the lead when Trossard struck at the back post four minutes after coming off the bench.
But Brentford got the reward their excellent away performance at Emirates Stadium deserved when Toney nodded in Christian Norgaard's cross from close range.
There was a lengthy check by the video assistant referee for a possible offside in the build-up but the goal was eventually confirmed.
Arsenal raised the tempo in the closing stages - including seven minutes of added time - as they searched for a winner but Brentford completed an excellent all-round defensive display by holding firm for a point.
Arsenal face Manchester City - six points behind before their game on Sunday afternoon against Aston Villa - in a top-of-the table showdown on 15 February.
Brentford, meanwhile, are eighth and just five points off fifth place.
Gunners title hopes dented by resilient Bees
Arsenal have been formidable at home this season, winning eight and drawing one of their nine games at the Emirates before the visit of Brentford.
They are one of two sides, along with Newcastle, in the top flight unbeaten on their own turf - but that record was given a stern test against in-form Brentford, who had won four of their previous five league games.
Mikel Arteta's side had plenty of possession but found little joy in unlocking a disciplined Bees defence their patience was rewarded when Trossard struck midway through the second half.
The Belgium forward has looked a fine addition since joining from Brighton & Hove Albion for £27m last month and he was close to adding a second soon after with a shot from distance.
But a one-goal lead rarely looked like it would be enough against a Brentford side who had been sharp in attack, and so it proved when Toney equalised.
The draw for Arsenal follows successive defeats in the league and FA Cup to Everton and Manchester City respectively.
How they respond to this relative stumble - not least in Wednesday's home game against City will be key in determining whether they can win a first Premier League title since 2003-04.
More to follow.
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Arsenal
Squad number19Player nameTrossardSquad number21Player nameFábio VieiraBrentford
Squad number1Player nameRaya
Squad number20Player nameAjer
Squad number5Player namePinnock
Squad number16Player nameMee
Squad number30Player nameRoerslev
Squad number27Player nameJanelt
Squad number6Player nameNørgaard
Squad number8Player nameJensen
Squad number3Player nameHenry
Squad number19Player nameMbeumo
Squad number17Player nameToney
Squad number9Player nameSchade
Squad number10Player nameDasilva
Squad number11Player nameWissa
Line-ups
Arsenal
Formation 4-3-3
1Ramsdale
4White12Saliba6Gabriel35Zinchenko
8Ødegaard5Partey34Xhaka
7Saka14Nketiah11Martinelli
1Ramsdale4White12Saliba6Gabriel35Zinchenko8Ødegaard5Partey34XhakaSubstituted forFábio Vieiraat 81'minutes7Saka14Nketiah11MartinelliSubstituted forTrossardat 62'minutesSubstitutes
3Tierney15Kiwior16Holding18Tomiyasu19Trossard20Jorginho21Fábio Vieira30Turner85Cozier-DuberryBrentford
Formation 3-5-2
1Raya
20Ajer5Pinnock16Mee
30Roerslev27Janelt6Nørgaard8Jensen3Henry
19Mbeumo17Toney
1Raya20Ajer5Pinnock16Mee30Roerslev27JaneltSubstituted forDasilvaat 71'minutes6NørgaardBooked at 90mins8Jensen3Henry19MbeumoSubstituted forWissaat 71'minutes17ToneySubstituted forSchadeat 90+7'minutesBooked at 90minsSubstitutes
2Hickey9Schade10Dasilva11Wissa13M Jorgensen23Lewis-Potter24Damsgaard26Baptiste34CoxReferee:Peter BankesAttendance:60,254Live Text
Match ends, Arsenal 1, Brentford 1.
Second Half ends, Arsenal 1, Brentford 1.
Kevin Schade (Brentford) is shown the yellow card for a bad foul.
Oleksandr Zinchenko (Arsenal) wins a free kick on the left wing.
Foul by Kevin Schade (Brentford).
Substitution, Brentford. Kevin Schade replaces Ivan Toney because of an injury.
Christian Nørgaard (Brentford) is shown the yellow card for a bad foul.
Oleksandr Zinchenko (Arsenal) wins a free kick in the defensive half.
Foul by Christian Nørgaard (Brentford).
Attempt blocked. Christian Nørgaard (Brentford) right footed shot from the left side of the six yard box is blocked. Assisted by Mathias Jensen with a cross.
Corner, Brentford. Conceded by Thomas Partey.
Delay over. They are ready to continue.
Delay in match because of an injury Ivan Toney (Brentford).
Foul by Ben White (Arsenal).
Ivan Toney (Brentford) wins a free kick in the defensive half.
Thomas Partey (Arsenal) wins a free kick in the defensive half.
Foul by Ivan Toney (Brentford).
Foul by Leandro Trossard (Arsenal).
Yoane Wissa (Brentford) wins a free kick in the defensive half.
Attempt missed. Bukayo Saka (Arsenal) left footed shot from outside the box is too high.
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