Investigations of the impacts of past volcanic eruptions on climate, environment and society require accurate chronologies. However, eruptions that are not recorded in historical documents can seldom be dated exactly. Here we use annually-resolved radiocarbon (14C) measurements to isolate the 775 CE cosmogenic 14C peak in a subfossil birch tree that was buried by a glacial outburst flood in southern Iceland. We employ this absolute time-marker to date a subglacial eruption of Katla volcano to late-822/early-823 CE. We argue for correlation of the 822/3 eruption with a conspicuous sulfur anomaly evident in Greenland ice cores, which follows in the wake of an even larger volcanic signal (circa 818-820 CE) as yet not attributed to a known eruption. An abrupt summer cooling in 824 CE, evident in tree-ring reconstructions for Fennoscandia and the Northern Hemisphere, suggests a climatic response to the Katla eruption. Written historical sources from Europe and China corroborate our proposed tree ring-radiocarbon-ice core linkage but also point to combined effects of eruptions occurring during this period. Our study describes the oldest, precisely dated, high-latitude eruption, and reveals the impact of an extended phase of volcanic forcing in the early-9th century. It also provides insight into the existence of pre-historic woodland cover and the nature of volcanism some decades before Iceland’s permanent settlement began.