The Italian, who went into the match undefeated, landed with a early right hook before the unified champion asserted his authority and dictated the tempo.
Klitschko kept his opponent at arm's length throughout the fight, peppering him with swift jabs, but Pianeta did manage to catch him on the jaw with a left in round three.
The big Ukrainian really began to dominate the contest in round four when he sent Pianeta to the canvas with a vicious right.
The challenger went down again under heavy punishment in the fifth round, but staggered to his feet as the referee's count reached eight.
Klitschko finished the job in the sixth round with a deft three-punch combo that sent Pianeta reeling, convincing the official to call time on the fight.