Messi is leaving the Nou Camp after 21 years, after failing to agree with fresh terms with the club.
Earlier in the day, the 34-year-old bid farewell in an emotional press conference.
But his teammates put his departure behind to crush Juve.
Memphis Depay opened the scoring after just three minutes. Martin Braithwaite made it 2-0 with a thumping header just before the hour mark from a corner.
Riqui Puig added a third in stoppage time with a curling left-foot drive.
The new LaLiga season gets underway on Friday, but Barcelona hosts Real Sociedad on Sunday in their opening fixture.