The iPad has taken just 28 days to pass the 1 million mark – more than twice as fast as the iPhone, according to Apple.
The much-hyped device reached the milestone on Friday, just as the first 3G-capable version went on sale.
However, demand for the “magical product” still outstrips supply, CEO Steve Jobs said in a statement.
The 3G iPad was sold out in many Apple stores by Sunday, with Piper Jaffray & Co. analyst Gene Munster estimating that 300,000 were sold over the weekend, the WSJ reported.
The device is available only in the US. International sales, originally planned for late April, will not start until at least the end of May.
Apple said since it went on sale on April 3, iPad users have already downloaded 12 million apps from the App Store and 1.5 million ebooks from the new iBookstore.
Meanwhile, the US Department of Justice and Federal Trade Commission are reportedly in discussions over a possible antitrust probe into Apple's new policy for software developers requiring them to use only Apple's programming tools in making iPhone apps.
The inquiry will examine whether the policy weakens competition by forcing programmers to choose between developing apps that can run only on Apple devices or come up with apps that are platform-neutral, New York Post reported.