The nonlinear and complex behavior of glacier dynamic processes (e.g., surging and ice calving) presents major challenges for future estimates of runoff and sea-level change. Because direct observations are temporally limited, reconstructions of past fluctuations from glaciers that undergo dynamic advance and/or retreat are valuable. We constructed a 300 yr, annually resolved record of surges and terminus fluctuations of Langjökull ice cap (central Iceland) using a novel combination of varve counting, multibeam bathymetry, seismic imagery, and multiple sediment cores from targeted sites in Hvítárvatn, a large proglacial lake. Langjökull achieved its maximum Neoglacial extent between ca. A.D. 1700 and ca. 1930, when two outlet glaciers, Norðurjökull and Suðurjökull, advanced into the lake and maintained active calving margins. Norðurjökull advanced into the basin ca. 1720, and remained at or near its maximum extension for most of the 19th century, whereas Suðurjökull underwent a quasi-periodic series of 8 surges between 1828 and 1930, with a recurrence interval of 14 ± 4 yr. Each surge event resulted in fragmentation of the glacier terminus during advances of up to1.6 km that occurred in <2 yr. Collapse of the expanded ice, iceberg melting, and reestablishment of the ice front at a nearshore grounding line occurred within 1–3 yr of the surge. Surges converted glacier ice to runoff at ∼10× the non-surging rate. Our precise estimates of the timing, duration, and magnitude of Suðurjökull surges provide ideal targets for the next generation of glacier surge models.