Increases in ¹⁴C concentrations in tree rings could be attributed to cosmic-ray events^,,,,,,, as have increases in ¹⁰Be and nitrate in ice cores^,. The record of the past 3,000 years in the IntCal09 data set, which is a time series at 5-year intervals describing the ¹⁴C content of trees over a period of approximately 10,000 years, shows three periods during which ¹⁴C increased at a rate greater than 3‰ over 10 years. Two of these periods have been measured at high time resolution, but neither showed increases on a timescale of about 1 year (refs and ). Here we report ¹⁴C measurements in annual rings of Japanese cedar trees from ad 750 to ad 820 (the remaining period), with 1- and 2-year resolution. We find a rapid increase of about 12‰ in the ¹⁴C content from ad 774 to 775, which is about 20 times larger than the change attributed to ordinary solar modulation. When averaged over 10 years, the data are consistent with the decadal IntCal ¹⁴C data from North American and European trees. We argue that neither a solar flare nor a local supernova is likely to have been responsible.