The win takes Maurizio Sarri’s men above Inter Milan into first place as the Nerazzurri are away at Napoli later in the day.
All four goals came in the second half as Juventus struggled to break down the visitors in the first.
The visitors played resolutely in the first half and refused to allow Juventus any clear cut chances in the opening period.
Former Arsenal midfielder Aaron Ramsey flashed a volley wide in the 12th minute following a cross by Adrien Rabiot.
The closest Juventus came to opening the scoring was when defender Merih Demiral rose highest to head Miralem Pjanic’s header against the crossbar ten minutes to half time.
But straight after the break all of Cagliari’s good work was undone when former Liverpool defender Ragnar Klavan gave the ball way in own area, allowing Cristiano Ronaldo to capitalize and round goalkeeper Robin Olsen for the opener.
Olsen came to Cagliari’s rescue in the 51st minute when he blocked Paulo Dybala’s shot with his legs before parrying away the rebound from teammate Klavan.
Cagliari came close to an equaliser on the hour mark but Giovanni Simeone’s header from Radja Nainggolan’s cross bounced off the cross bar.
The home side were awarded a penalty in the 65th minute after Dybala went down in the box under the challenge of Marko Rog, allowing Ronaldo to step up and send the goalkeeper the wrong way for his second of the game.
Ronaldo then turned provider when he played substitute Gonzalo Higuain through, and the Argentine cut inside Klavan before hitting a deflected effort past Olsen in the 80th minute.
The five-time Ballon d’or winner completed his hattrick nine minutes from time when he fed on substitute Douglas Costa’s pass to slot the ball under the advancing Olsen.
