Solar-mass stars form via disk-mediated accretion. Recent findings indicate that this process is probably episodic in the form of accretion bursts(1), possibly caused by disk fragmentation(2-4). Although it cannot be ruled out that high-mass young stellar objects arise from the coalescence of their low-mass brethren(5), the latest results suggest that they more likely form via disks(6-9). It follows that disk-mediated accretion bursts should occur(10,11). Here we report on the discovery of the first disk-mediated accretion burst from a roughly twenty-solar-mass high-mass young stellar object(12). Our near-infrared images show the brightening of the central source and its outflow cavities. Near-infrared spectroscopy reveals emission lines typical for accretion bursts in low-mass protostars, but orders of magnitude more luminous. Moreover, the released energy and the inferred mass-accretion rate are also orders of magnitude larger. Our results identify disk-accretion as the common mechanism of star formation across the entire stellar mass spectrum.