Pep Guardiola's side came into the match six points behind Madrid following the challengers' win against Granada on Sunday but were hoping to cut the gap back to three by beating their lowly neighbours.
They looked on course to do so as Cesc Fabregas gave them a 16th-minute lead but after failing to build on the opener, Barca - who also had efforts correctly ruled out for handball and offside - conceded the equaliser with less than five minutes to go as Alvaro Vazquez dived to head home.
Gerard Pique hit the bar with a late effort and Espanyol survived a penalty appeal for handball as Barca were left frustrated.