Four months after Juventus eliminated them in the quarterfinals, Barcelona began the group stage with a victory at the Camp Nou as Messi scored just before the break and helped set up a second before adding Barca's third himself.
Ousmane Dembele, making his first Barcelona start since his €150 million move from Dortmund this summer, nearly broke through after intercepting a foolhardy pass from Mattia De Sciglio in the box, but he could only fire wide.
De Sciglio had to come off with a knock soon afterwards, and Messi provided the opener just before the break by rolling a shot inside the far post after a one-two with Luis Suarez.
Messi nearly had a second goal soon after the break, but his shot from 20 yards shook the post, then came back off of Gianluigi Buffon before falling out of play.
The Argentine then played a large role in Barcelona's second as he ran the ball to the endline, and his cross across the face of goal was only cleared so far as the middle of the box, where Ivan Rakitic was on hand to lash home.
Messi then ran past two stationary Juventus defenders to enter the box and add his second goal in the 69th minute.