Leganes stuns Barcelona ahead of Atleti showdown
Robert Lewandowski came close to restoring parity seven minutes later, but LaLiga's leading marksman was unable to turn Raphinha's cross beyond a brilliant Marko Dmitrovic in the Leganes goal.
But the hosts' best chance of an equaliser came in the closing stages, with Ferran Torres neatly flicking the ball into Kounde's path, but he dragged his effort past the post.
Barcelona remains top of the standings, but it level on points with its opponent for next week, Atletico Madrid, and only a point ahead of Real Madrid, which like Atletico has a game in hand over the Blaugrana.
Barca has now suffered consecutive home defeats for the third time in all competitions in the 21st Century, after April 2003 under Radomir Antic (two) and April 2022 with Xavi (three).
After Atletico earned a narrow victory and with Carlo Ancelotti's Madrid also dropped points, Barcelona missed the opportunity to stretch a lead that has been narrowing in recent weeks at the summit.
Hansi Flick's side ended the contest with an expected goals (xG) total of 2.67 from their 20 shots, though only four were on target, while they also managed to restrict Leganes to just six attempts, but the visitors matched their opponents' total for efforts on goal.
Adding to its frustration, Barcelona enjoyed 80 per cent of possession against Leganes, the second-highest by a team in a home defeat in LaLiga since 2005-2006.