In their earliest stages, protostars accrete mass from their surrounding envelopes through circumstellar disks. Until now, the smallest observed protostar-to-envelope mass ratio was about 2.1 (ref. 1). The protostar L1527 IRS is thought to be in the earliest stages of star formation. Its envelope contains about one solar mass of material within a radius of about 0.05 parsecs (refs 3, 4), and earlier observations suggested the presence of an edge-on disk. Here we report observations of dust continuum emission and (13)CO (rotational quantum number J = 2 → 1) line emission from the disk around L1527 IRS, from which we determine a protostellar mass of 0.19 ± 0.04 solar masses and a protostar-to-envelope mass ratio of about 0.2. We conclude that most of the luminosity is generated through the accretion process, with an accretion rate of about 6.6 × 10(-7) solar masses per year. If it has been accreting at that rate through much of its life, its age is approximately 300,000 years, although theory suggests larger accretion rates earlier, so it may be younger. The presence of a rotationally supported disk is confirmed, and significantly more mass may be added to its planet-forming region as well as to the protostar itself in the future.