The 40-year-old Klitschko used his considerable size advantage to keep the 34-year-old Adamek at bay with his powerful jab and was in complete control throughout the bout. Adamek stayed on his feet but his face was bruised and bloodied by the time Italian referee Massimo Barrovecchio stepped in to stop the punishment two minutes, 20 seconds into the 10th round.
"He's a very good boxer," Klitschko said. "He was a cruiserweight champion, he was a light heavyweight champion, but he's not good enough to be a heavyweight champion because it's a whole other league."
The 40-year-old Klitschko was six inches taller than his opponent and used his superior reach to have Adamek staggering back in the second, sixth and ninth rounds.
Klitschko slipped in the eighth, but carried on where he left off in the ninth and 10th to improve his record to 43-2, including 40 knockouts. Adamek drops to 44-2.
The win means Klitschko and his younger brother, Wladimir, retain their hold on all three major heavyweight titles.
"I think the referee made the right decision. He probably could have stopped it a minute earlier," said Wladimir, who holds the IBF and "super" WBA titles as well as the minor WBO and IBO belts.
It was the first heavyweight fight held in Poland, and Adamek was cheered on by a majority of the 42,000 spectators at Wroclaw's sold-out soccer stadium, which was built for next year's European Championship.
Klitschko weighed in for his 15th title fight at 242.5 pounds — the lightest he's been since 1998 — but still towered over his opponent, and the 216-pound Adamek simply couldn't reach him.
Adamek tried to force the issue in the second round but was caught by a huge right hook that left him wobbling against the ropes and the crowd fearing an early outcome.
Adamek recovered slightly in the fourth, despite a bloody nose and mouth, but failed to really trouble Klitschko.
Barrovecchio checked that Adamek was fit to continue after taking a powerful left-right combination in the sixth, and Klitschko's left foot gave way while pushing Adamek into the corner in the eighth.
"I twisted my foot and lost my balance," he explained.
But the outcome was never in doubt, and Klitschko again had Adamek staggering after delivering a straight left jab to the head in the ninth.
It was only a matter of time before Adamek's challenge was brought to a halt.
"He was simply the better fighter and he won," Adamek said. "That's sport. You win some and you lose some. This time I lost."