Amazon's app store will sell Android apps, which can be bought either through smartphones or a computer.
Users must install the Amazon App Store app before they make a purchase.
At launch, the site is offering users a free version of the new 'Angry Birds' game – believed to be a favourite of Barack Obama. The original game has been downloaded more than 100 million times.
The Amazon version will be "curated" and policed for quality, unlike the Google store, which is open to any developer who wishes to publish apps. Last month some had to be pulled when they were found to contain "malware".
Users will also be able to take their apps with them if they change handset – a policy that was first trialled with Kindle purchases.
Last week, the app web page was launched early, and reports suggest some 50 apps could be viewed.
Reports also suggest that any purchases must be processed through Amazon's payments system, with no links to outside sites, such as Google's Apple Store.
Apple has filed a suit to sue Amazon for using the term "App Store" without permission in the US. Filing in California, Apple has asked the court to prevent Amazon using the term.
Android now accounts for more than 50% of US mobile ad impressions, due to the phenomenal rise in the popularity of the platform.