The Ukrainian was never troubled by Pianeta, finishing off the previously unbeaten Italian southpaw with a right-left combination. Pianeta stayed down before attempting to continue, but there was no point in delaying the inevitable and referee Ernie Sharif ended the bout 2:52 into the round.
"Francesco tried. He has a fighting heart," Klitschko said. "He's a positive example in boxing. I'm sure he's learned a lot and that he has a great future ahead of him."
Klitschko rocked Pianeta inside the first 30 seconds, knocked him to the canvas with a straight right down the middle in the fourth, and again with a left blow in the fifth.
The 37-year-old Klitschko improved to 60-3 after his 52nd knockout, while Pianeta dropped to 28-1-1.
"I'm very disappointed and sad," said Pianeta, a cancer survivor who lives in Gelsenkirchen, Germany. "I can't say anything. It wasn't my day. He's the world champion. It wasn't enough. That was rubbish today."
The fighters worked together briefly last year and Klitschko tried to be kind to his former sparring partner.
"Francesco wasn't easy to box," Klitschko said. "He punched with the hardest punches I've ever felt. Luckily, they didn't hit me."
Vitali Klitschko, who holds the WBC belt, said he had no worries ahead of his younger brother's 23rd championship bout.
"My brother was very strong," Vitali Klitschko said.
Next up for the younger Klitschko is a likely bout with mandatory challenger Alexander Povetkin of Russia, the "regular" WBA champion after Klitschko was elevated to "super champion" status.