The open, first Grand Slam event of the year, begins Jan. 19 in Melbourne.
"I am very sorry to announce that I am not going to be able to defend my title at this year's Australian Open," Sharapova said on her Web site. "My shoulder is doing great, but I just started training a few weeks ago and I am just not near the level I need to be to compete at the highest levels."
The Russian star won her third Grand Slam title at Melbourne last year, winning every set she played.
"The Australian Open is one of my favorite tournaments; they have the best fans in the world, and I am going to miss everything about the tournament. I promise I will see everyone next year," Sharapova said.
Sharapova was forced out of the Summer Olympics and did not play in the U.S. Open in 2008 because of the shoulder problem.
"I know she has been working around the clock to try and get back from the shoulder injury," said Craig Tiley, director of the open. "In the end, time beat her this year."