Klitschko knocked out European heavyweight champion Sosnowski, 21, at the Veltins Arena, home of Bundesliga football side Schalke 04, and this was the fourth defence for the 38-year-old of the WBC title he won in October 2008.
"I was in good form, Sosnowski was younger than me, but people saw that I did well against him," said Klitschko. "He did everything to win this fight, but my experience got me through."
Klitschko said his next opponents are likely to be Britain's WBA heavyweight champion David Haye or former champ Ruslan Chagaev.
"I am a bit disappointed, because I thought I had a good chance of winning this fight," said Sosnowski, who recovered from the punch which sent him slumping to the canvas.
"I am okay now, it was a hard punch, but I was also quite tired and I just didn't have the energy to get up."
Keeping his guard low, Klitschko repeatedly pawed the Polish challenger in the opening rounds with Sosnowski, 13 centimetres shorter and 23 pounds lighter than his opponent, looking to land early punches on the champion.
Klitschko was not looking as physically prepared as usual and stayed mainly in the centre of the ring allowing the challenger to connect with the Ukrainian's chin in the fourth round.
The Ukrainian started to make his advantage count with a combination of punches in the fifth round and again in the sixth.
Sosnowski started landing some telling punches and made some progress against the champion in the seventh and eighth rounds as Klitschko seemed to tire.
But the Ukrainian finished the ninth round strongly, caught Sosnowski across the face and then pinned him to the ropes with a flurry of punches, but the bell saved the Polish fighter.
Sosnowski signalled to the crowd, but he looked wobbly at the start of the tenth and a big right opened a cut above his left eye and sent him slumping to the canvas with 29 seconds left of the round.