Amazon Web Services (AWS) is a cloud computing offering that makes money by charging businesses to host websites and other applications.
Amazon founder Jeff Bezos said in a statement: "Amazon web services is a $5bn business and still growing fast."
The firm's total revenues for the quarter rose by 15% to $22.7bn.
The increase was stronger than expected, with revenues buoyed by increased sales in North America, Amazon's biggest market. Despite the rise, the company reported a loss of $57m for the quarter.
Shares in the firm rose nearly 5% in after-hours trading.
The AWS division provides cloud computing services to household names including Dropbox, Spotify, Netflix, Uber, Samsung and even the CIA - helping them send notifications, stream video and synchronise data.
The figures for the first time confirm that Amazon's cloud business is the biggest of its kind in terms of revenue.