Okafor finished just ahead of Chicago's Ben Gordon for the honor.
Gordon helped push Chicago into the playoffs for the first time in seven seasons while Okafor's team was never a playoff factor. They were teammates at the University of Connecticut.
Okafor averaged 15.1 points and 10.9 rebounds a game this season. He had 47 double-doubles.
"This is an award that every rookie strives to get and it means a lot to me," Okafor said.