The Spanish No.1 has put aside the fierce rivalry between the two clubs to praise the Catalans, insisting that the recent El Clasico animosity "will be forgotten in time"
Pep Guardiola's men drew 1-1 with Levante on Wednesday evening which was enough for the club to win their third successive domestic title.
Rivalry between Real and Barcelona has been fierce in recent times with the two team's meeting on four occasions within a month. However, Casillas has chosen to put the tension aside momentarily to commend the Catalan club.
"I have to give Barca my congratulations," he said. "[To win] Championships are very tough and you have to be good. I congratulate them on my behalf and on Real Madrid's.
"There is no need for controversy. They were friendly when we won the [Copa del Rey] cup and we are the same now."
The Spanish No.1 is team-mates with a host of Barcelona players for the national side and wanted to make it clear there is no bad blood between himself and the champions.
"I'm happy for the friends I have there," he added. "I get on with all of the Barcelona players who I know. I understand there was controversy after the four Clasicos but all will be forgotten in time."