Mehdi Taremi and Moussa Marega got on the scoresheet for Porto, which opened each half with an early goal to grab a stunning lead in the two-legged round-of-16 matchup.
Cristiano Ronaldo was held scoreless Wednesday at Estadio do Dragao in his home country of Portugal, leaving Juventus with an uphill battle to advance.
The trip to the quarterfinal will be decided in the second leg March 9 at Juventus Stadium in Turin, Italy.
The first leg couldn't have began any worse for Juve, which conceded the opening goal in just over a minute.
Rodrigo Bentancur was trying to pass it back to the keeper but it was Mehdi Taremi who found the ball for the quick score.
The second half saw a similar start as Moussa Marega scored for Porto in a matter of seconds after intermission.
Wilson Manafa dribbled through the defense and no one marked Marega for what became an easy goal.
While the defensive mistakes contributed to the goals, Juventus had nothing going offensively either with little creativity in the final third.
Carlo Garganese @carlogarganese
Juventus have had some terrible centre midfields since 2016…this one tonight may be the worst yet Bentancur, Rabiot & McKennie unwatchable 1st half. Zero creativity. So easy for Porto. They could play til Xmas & not concede Pirlo needs to change something, no idea what #PORJUV
Porto controlled only 35 percent of possession but they were happy to stay back with a compact line to prevent any looks at the net.
The Italians only had one shots on goal in the first half and three in the first 80 minutes before a late charge.
Federico Chiesa came through with a clutch goal for Juve in the 82nd minute to keep hope alive.
The Serie A club couldn't complete the comeback, but the away goal could be extremely valuable going into the second leg.
Porto is sometimes overlooked in this event compared to the clubs from the bigger leagues across Europe, but the squad showed Wednesday it can compete with the best in the world. Juventus will need to improve in all areas in order to survive this difficult test.