The Blaugrana moved to 85 points from 34 matches and extended their lead at the summit to an unassailable 14 points over second-placed Real Madrid.
There was little time for jubilation, however, as the visitors had to quickly rush off the pitch after fulltime as several Espanyol fans invaded the field as Barca started to celebrate their title.
They will have to wait until Saturday to lift the LaLiga trophy as they host fourth-placed Real Sociedad in front of their jubilant fans who will pack out their Camp Nou stadium.
It will no doubt be an emotional occasion for captain Sergio Busquets, who last week announced that he will be leaving Barca after 15 years at the end of the season.
It was Busquets’ ninth LaLiga title and 32nd overall with the club, with his trophy cabinet also including three Champions Leagues, seven Copa del Rey and three FIFA Club World Cups as well as his 2010 World Cup and 2012 European championship success with Spain.
Barca were the superior side against lowly Espanyol, who are second from bottom and fighting to avoid relegation, and opened the scoring early with a close-range strike by Lewandowski.
Alejandro Balde, who put it on a plate for Lewandowski’s opener, scored Barca’s second with a tap-in from a Pedri cross in the 20th minute.
Lewandowski extended Barca’s lead five minutes before the break with another close-range finish from a Raphinha cross.
Defender Jules Kounde headed in Barca’s fourth in the 53rd minute, before Javi Puado and Joselu scored consolation goals for Espanyol.
Barcelona have won 27 of their 34 games this season, with their title charge built largely on their outstanding defence, which has conceded a measly 13 goals, by far the best defensive record in Europe’s top five leagues. (Reuters)