Amazon did not disclose actual sales numbers, but a spokeswoman for the company said the top-seller status was based on unit sales.
Amazon Chief Executive Jeff Bezos recently told the BBC that it sells the tablets at cost, hoping to generate profits when customers use the devices to buy products and services from the company.
The announcement came as Apple Inc prepares to launch a smaller, cheaper tablet of its own on Tuesday. The device, dubbed the iPad Mini, may challenge Amazon's success at the lower end of the booming tablet market.
Meanwhile, Amazon has launched larger, more expensive Kindle Fire HD tablets, aimed at Apple's full-sized iPad, which still dominates the sector.