The online retailer said Wednesday that the self-created holiday was its biggest sales day ever, with worldwide orders rising more than 60 percent compared with the previous Prime Day. In the U.S., orders rose by more than 50 percent.
The company declined to share how many people signed up for Prime to participate in the sale. To qualify for the sales event, which was created last year to celebrate Amazon's 20th birthday, shoppers need to be members of the service.
"The numbers blew me away," R.J. Hottovy, Morningstar's consumer strategist, told CNBC. He estimates the event may have added between $500 million and $600 million in incremental sales to the retailer's topline.
FBIC analyst Deborah Weinswig estimates sales at $525 million, up 26 percent from their projected sales of $415 million a year ago.
Analysts attributed the lift in Amazon's order volume to several factors. For one, it stocked its virtual shelves more deeply, allowing more customers to buy the deals they wanted, R.W. Baird analyst Colin Sebastian said. For another, it rolled out new discounts as often as every five minutes, compared with every 10 minutes last year.
"Naturally if you increase the number of offers you have substantially, you're also going to increase sales volume substantially," Forrester analyst Sucharita Mulpuru told CNBC. Given that Amazon accounts for roughly one-third of Cyber Monday's $3 billion in U.S. sales, Mulpuru "wouldn't be surprised" if the event was a billion-dollar day for Amazon, she said.
"That's significant, but it's one day," she said, adding that some purchases of big-ticket items may have come from shoppers who were already planning to buy those things a later date. Still, there were likely a fair amount of sales stolen from competitors, as well as unplanned impulse purchases on lower-price items.
Amazon shares closed slightly lower, at $742.63, Wednesday. That added to losses Tuesday, which began after a midday sales update from ChannelAdvisor. The e-commerce software company, which helps power the online operations of roughly 3,000 retailers, said Prime Day sales for its customers selling on Amazon were flat with the prior year's event.
But Piper Jaffray analyst Gene Munster cautioned investors about overreacting to that data point, as ChannelAdvisor's numbers represent just a sample of Amazon's sellers, he said. He added that the figures only capture same-store sales trends; and given that the number of vendors participating in Prime Day was more than double the prior year, they did not capture the complete picture.
Meanwhile, Amazon spread out its deals further into the day, including more than 800 of its limited-time "Lightning Deals" launching well into the evening. The retailer said Prime members saved more than double the amount they did last Prime Day — and it is already working to amp them up for next year's event.
"After yesterday's results, we'll definitely be doing this again," Greg Greeley, vice president of Amazon Prime, said in a press release announcing the results.
On its inaugural Prime Day, Amazon sold more units than it did on the previous Black Friday, which at the time was its largest sales day ever. The company did not provide an explicit sales forecast for this year's event, but said it anticipated the "record-breaking" day it received.
Other retailers hoped to capitalize on Prime Day's buzz by launching their own promotions. That includes Wal-Mart, which eliminated its $50 free shipping minimum this week. But the world's largest retailer was mum on Tuesday's results.
"While we aren't breaking out sales or providing updates outside of quarterly earnings, I can tell you millions of customers have been saving all month long on Walmart.com. Yesterday was no exception," Wal-Mart spokesman Ravi Jariwala said in a statement to CNBC.
Though analysts predicted other retailers would once again see a lift in traffic on Prime Day, Steve Osburn, a director at Kurt Salmon, said competitors' deals didn't stack up.
"They don't have nearly the product availability," he said.
Amazon sold more than 90,000 TVs and more than two million toys on Prime Day, the company said. Ordering on its mobile app also accelerated compared with the prior year, increasing more than two times among Prime members.
But the day was not without speed bumps. Just a few hours into the sale, Amazon shoppers reported issues adding items to their digital shopping carts. The problem was resolved a few hours later. Amazon would not comment on whether that hiccup caused it lose out on sales, though Munster was skeptical that it hindered the retailer's performance.
"The results indicate that the Amazon Prime brand remains strong and user engagement continues to improve," he told investors. Amazon's 60 percent unit growth topped Piper Jaffray's baseline expectations for 37 percent growth, and likely added 0.5 percent of incremental unit growth in the third quarter.
The retailer's July sales event is about more than generating revenue in an otherwise slow shopping month. The bigger goal is to encourage shoppers to sign up for a Prime subscription, which offers free two-day shipping and other perks for members. A monthly membership costs $10.99; an annual membership is $99. Last year's event led to more people trying Prime than any day in the site's history, Amazon said. But this year, it did not provide details on signups.
By getting shoppers to enroll in this service, it encourages them to spend more money with Amazon. Whereas Prime members spend an estimated $1,200 a year on Amazon, non-members spend roughly $500, according to Consumer Intelligence Research Partners.
Once Amazon persuades shoppers to sign up for the service, they tend to be loyal. Separate data from CIRP has found that people who sign up for a free 30-day trial are converted into paying members 73 percent of the time. And after one year, 91 percent of members renew for a second.
As the pool of Prime members grows, however, it will be increasingly difficult for it to attract new signups. Amazon does not specify the number of Prime members, but CIRP estimates roughly 63 million people in the U.S. are part of the program. A separate estimate from Cowen & Co. analyst John Blackledge pins the figure at closer to 45 million.
Both firms estimate that Prime shoppers account for roughly half of Amazon's purchases.
For more, here's the full press release from Amazon: