King's Cup final against Atletico Madrid, Real's last chance for major silverware this season, giving the occasion a sense of anti-climax.
Cristian Stuani gave Espanyol the lead after 28 minutes, and although Gonzalo Higuain pulled one back for Real in the 58th it was not enough to stop Barca securing the league trophy.
Barcelona have 88 points with four games left to play. Real have 81 in second having played two matches more than their arch-rivals.
“First I want to congratulate Barcelona, their coaching staff, their players, directors and fans,” said Real manager Mourinho. “They have won a league title which was ours until now and I think the least we can do is congratulate them.”
Despite the La Liga crown being at stake, Real started with the likes of Cristiano Ronaldo, Xabi Alonso and Karim Benzema on the bench and mid-table Espanyol, who have little left to play for this season, sniffed the chance for an upset.
The game took a turn for the worse for Real when Raphael Varane fell awkwardly as he made a tackle and went down clutching his right knee.
The promising young French defender had to be substituted after 19 minutes, and in the 28th Stuani pounced on a loose ball in the area at a corner to fire the hosts in front.
Espanyol were unlucky to have a second goal ruled out soon after, while Real offered little going forward until Higuain flicked a header inside the far post from Luka Modric’s freekick just before the hour mark.
The equaliser came two minutes after Real's leading scorer Ronaldo had entered the fray and the visitors steadily increased the pressure on the Espanyol goal althoughthey rarely looked like finding the winner.
After title, Barca shift attention
Barcelona were turning their attentions to strengthening the squad for next season after winning their fourth La Liga title in five years.
“There’ll only be a few changes,” Barcelona vice-president Josep Bartomeu said. “It’s a very competitive team and it’s very difficult to substitute the pieces. (Spain goalkeeper) Victor Valdes has told us he doesn’t want to continue, so we will try to find a replacement this summer. Also a central defender and a forward.”
“If we look at our squad, 80 percent of the players that would interest us are already part of our team,” added sports director Andoni Zubizarreta.