Dust-emission observations of the young (<500,000 years old) protostar IRS 63 show evidence of rings and gaps in its disk, a prerequisite of planet formation. Annular structures (rings and gaps) in disks around pre-main-sequence stars have been detected in abundance towards class II protostellar objects that are approximately 1,000,000 years old(1). These structures are often interpreted as evidence of planet formation(1-3), with planetary-mass bodies carving rings and gaps in the disk(4). This implies that planet formation may already be underway in even younger disks in the class I phase, when the protostar is still embedded in a larger-scale dense envelope of gas and dust(5). Only within the past decade have detailed properties of disks in the earliest star-forming phases been observed(6,7). Here we report 1.3-millimetre dust emission observations with a resolution of five astronomical units that show four annular substructures in the disk of the young (less than 500,000 years old)(8)protostar IRS 63. IRS 63 is a single class I source located in the nearby Ophiuchus molecular cloud at a distance of 144 parsecs(9), and is one of the brightest class I protostars at millimetre wavelengths. IRS 63 also has a relatively large disk compared to other young disks (greater than 50 astronomical units)(10). Multiple annular substructures observed towards disks at young ages can act as an early foothold for dust-grain growth, which is a prerequisite of planet formation. Whether or not planets already exist in the disk of IRS 63, it is clear that the planet-formation process begins in the initial protostellar phases, earlier than predicted by current planet-formation theories(11).