The annual cycle and secular trend of Greenland mass loading are well recorded in measurements of solid Earth deformation. Horizontal crustal displacements can potentially track the spatiotemporal detail of mass changes with great fidelity. Our analysis of Greenland crustal motion data reveals that a significant excitation of horizontal amplitudes occurs during the intense melt years. We discover that solitary seasonal waves of substantial mass transport (1.67 ± 0.54 Gt/month) traveled at an average speed of 7.1 km/month through Rink Glacier in 2012. We deduce that intense surface melting enhanced either basal lubrication or softening of shear margins, or both, causing the glacier to thin dynamically in summer. The newly routed upstream subglacial water was likely to be both retarded and inefficient, thus providing a causal mechanism for the prolonged ice transport to continue well into the winter months. As the climate continues to produce increasingly warmer spring and summer, amplified seasonal waves of mass transport may become ever more present with important ramifications for the future sea level rise. Plain Language Summary It has become well known that seasonal ice flow variability of Greenland outlet glaciers may often be associated with the drainage of supraglacial lakes that accumulate meltwater during summer. However, tracking the details is inevitably limited due to the fact that the mechanisms and rates of meltwater transfer are hidden from view, and theoretical models are fraught with a number of difficulties. Here we use a previously unrecognized source of data that constrain the mass transport during a season of intense Greenland melting and document the evolution of a mass transport wave as it passes down glacier. The breakthrough is twofold: demonstration of the power of the new technique and the first measurement of the mass amplitude. The technique is effectively using the measurement of the deformed solid Earth elastic response as a filter that uniquely responds to neighboring glacier mass changes. We quantify that the wave through Rink Glacier is enormous in terms of its mass transport, amounting to about half of the average annual discharge during 2000–2005, and travels at an average speed of 7.1 km/month. Our mass transport wave measurement is the first of its kind, on any of the major outlet glaciers of either Greenland or Antarctica.