AWS "was surprisingly more profitable than forecast", Dan Kurnos, an analyst at the Benchmark Company, told the BBC.
That "should help [Amazon] justify their heavy investment spending and provide a clearer path to profitability for the [overall] company as AWS grows," he added.
Michael Pachter, from Wedbush Securities said: "I think most of us believed that the business [AWS] was breakeven at best, and it is surprising that it generates such a significant portion of profit."
"The stock is up because it is clear that if that business scales, Amazon can be immensely profitable," he added.