This paper presents the first annually resolved temperature reconstruction for England in the Middle Ages. To effect this reconstruction the starting date of the grain harvest in Norfolk has been employed as a temperature proxy. Using c. 1,000 manorial accounts from Norfolk, 616 dates indicating the onset of the grain harvest were extracted for the period 1256 to 1431 and a composite Norfolk series was constructed. These data were then converted into a temperature series by calibrating a newly constructed comparison series of grain harvest dates in Norfolk from 1768 to 1816 with the Central England Temperature series. These results were verified over the period 1818–1867. For the British Isles no other annually resolved proxy data are available and the onset of the grain harvest remains the only proxy for assessing April-July mean temperatures. In addition, this is the first time-series regarding the onset of grain harvests in medieval Europe known so far. The long-term trend in the reconstructed medieval temperature series suggests that there was a cooling in the mean April-July temperatures over the period 1256 to 1431. Average temperatures dropped from 13°C to 12.4°C, which possibly indicates the onset of the Little Ice Age. The decline in values was not steady, however, and the reconstruction period contains decades of warmer spring-early summer temperatures (for example the 1320s to the early 1330s and the 1360s) as well as colder conditions (for example the late 1330s, 1340s and the 1380s). The decline in grain-growing-season average temperatures would not have been a major problem for medieval agriculture, rather the phases of very high interannual variability partly found in the medieval time-series, such as 1315–1335 and 1360–1375, would have proved disruptive.