The Italian-born challenger briefly ruffled the champion by landing a couple of punches in the second round but apart from that appeared out of his depth.
Klitschko twice sent his opponent to the canvas in the fourth round before the referee called a halt to proceedings eight seconds from the end of the sixth as Pianeta took a pounding.
The 37-year-old Ukrainian extended his unbeaten run to 18 fights, his last defeat coming when he challenged Lamon Brewster for the WBO title in 2004.
Overall, he has won 60 of his 63 fights, including 52 within the distance.
Klitschko holds the IBF, IBO, WBO and WBA crown and his older brother Vitali holds the WBC crown. The two have pledged not to fight each other.
Pianeta, who at 1.93 meters is five centimeters shorter than his opponent, was previously undefeated with 28 wins and one draw.
"He started well and he tried to take the fight to me, he fought with great courage," said Klitschko.
(Reporting by Brian Homewood, editing by Alan Baldwin)
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