The Keystone State was called by four media outlets for the Democratic standard-bearer shortly after polls closed.
Both parties targeted Pennsylvania and its 21 electoral votes as key to their march the White House.
McCain and his running mate, Alaska Gov. Sarah Palin, spent much time and money in the last weeks of the campaign in Pennsylvania, which his campaign targeted for victory, despite the state's historic Democratic leaning and polls that indicated Obama held a steady, double-digit lead.
When CNN announced its projection that Pennsylvania was in the Obama column, huge cheers and applause broke out in Chicago's Grant Park, where Obama's election night rally was being held.
A McCain spokesman told CNN the Arizona Republican's campaign hoped, but didn't expect to win Pennsylvania.
McCain is watching election returns at the Biltmore Hotel in Phoenix.