The three-time Grand Slam title winner already had announced she's pulling out of the Beijing Olympics because of the injury. Sharapova said in a posting on her Web site Friday she probably won't need surgery and could be ready to play in two to three months.
"It hurts me so much to miss the Olympics and the U.S. Open, you have no idea," she said. "Just to type those words hurt!!"
Earlier Friday, a U.S. Open official told The Associated Press that Sharapova's agent informed the tournament she wouldn't be able to play in the year's last Grand Slam event.
The No. 3-ranked Sharapova has played in each of the past 23 major championships, winning titles at Wimbledon in 2004, the U.S. Open in 2006 and the Australian Open in January.
A doctor who looked at tests on her shoulder from April and this week told Sharapova she has been playing with a torn rotator cuff tendon.
"He actually couldn't believe that I've been playing this long with this injury. You can imagine that I was not very thrilled to hear that my medical team did not see this tear in my shoulder back in April," she said. "The good news is that it didn't get much worse since April, but we could have started the healing time back then instead of now."
Sharapova will go to Arizona to work with a specialist for rehab and strength work.
The Olympic tennis tournament begins Aug. 11, and the U.S. Open starts Aug. 25.
Sharapova is 32-4 with three titles in 2008, and she briefly held the No. 1 ranking. She pulled out of the WTA tournament in Montreal because of the shoulder on Wednesday -- after winning a nearly 3-hour match in which she double-faulted 17 times.
The right shoulder injury also forced Sharapova to pull out of a tournament in March.
"Once I'm healthy, I'm sure I'll look at this as a blessing in disguise," she said on the Web site Friday. "Right now, it's a bit painful, of course, but every athlete goes through these patches, and I'm just grateful that this isn't as bad as it could have been."