The firm's revenue fell 5% from a year ago to about $84.3bn (£64.5bn) in the most recent quarter, with iPhone sales down 15% to about $52bn.
The stumble had been expected after the tech giant warned investors earlier this month that revenue would be about $84bn, lower than expected.
The firm had blamed the issues partly on an economic slowdown in China.
Executives have said they remain confident that growth in its services business, which includes Apple Pay and the App store, will drive future performance.
Revenue from the services business jumped 19% to a record $10.9bn in the quarter, which ended on the 31 December.
"While it was disappointing to miss our revenue guidance, we manage Apple for the long term, and this quarter's results demonstrate that the underlying strength of our business runs deep and wide," said Tim Cook, Apple's chief executive.