Leganes beat Barcelona for only the second time in their nine La Liga meetings
Barcelona suffered a surprise defeat at home to Leganes to see their lead at the top of La Liga cut.
Hansi Flick's side are only above Atletico Madrid on goal difference and one point above third-placed Real Madrid.
Leganes, who move up to 15th, took the lead in the fourth minute when Sergio Gonzalez headed in from Oscar Rodriguez's corner.
That would prove to be the only goal of the game.
Barcelona dominated, with 20 shots in total, and the closest they came was when Raphinha's half-volley from Alex Balde's cross hit the bar.
Earlier on Sunday Atletico beat Getafe 1-0 to move level on points with Barcelona.
Champions Real drew 3-3 at Rayo Vallecano on Saturday.
