Oscar De Marcos’ 41st-minute effort at the Nou Camp looked set to consign the Catalan giants – who suffered a shock 2-1 loss to Leganes in midweek – to back-to-back league defeats for the first time in over two years.
However, Barca, after hitting the woodwork through Philippe Coutinho and Lionel Messi, salvaged a 1-1 draw thanks to Munir’s leveller seven minutes from time.
Despite the late leveller, the result caps a disappointing week for the early-season pace-setters, who were also held at home by Girona last weekend before their Leganes upset.
Barca coach Ernesto Valverde opted to start with star man Messi on the bench ahead of a daunting run of fixtures that sees them face Tottenham, Valencia, Sevilla, Inter Milan and Real Madrid in succession.
But Athletic are no pushovers either and without their Argentinian talisman, Barca found themselves trailing at the break.
The visitors twice went close through Inaki Williams before Luis Suarez spurned a great chance for Barca in the 25th minute, the Uruguayan failing to beat Unai Simon with only the Athletic goalkeeper to beat.
To add to Suarez’s frustration, the Basque side made the most of that escape by taking the lead through De Marcos just before the break.
Athletic won possession deep in Barca territory and Markel Susaeta floated a cross to the far post which the airborne De Marcos volleyed past Marc-Andre ter Stegen.
Valverde threw on experienced duo Messi and Sergio Busquets at the start of the second half and was almost rewarded with an equaliser in the 62nd minute, although it was Coutinho and not the new arrivals who sent an effort against the bar.
Messi also struck the frame of the goal as Barca turned up the heat, the Argentinian firing against the post in the 76th minute after linking up with Jordi Alba.
But with time running out, it was beginning to look as though Athletic would be the first visiting team to win a league game at the Nou Camp since Alaves in September 2016.
It would also have been Athletic’s first LaLiga victory at this ground since 2001. But there was to be a late twist.
Munir had only been on the pitch four minutes since replacing Ousmane Dembele but he did what fellow replacement Messi was unable to by drawing Barca level.
Simon could only parry Messi’s shot and when the Argentina star pounced on the rebound before drilling the ball back into the six-yard box, Munir was in the right place to stick out a foot to divert it home.