On the basis of 25 years of digitized weekly sea surface temperature (SST) maps of the North Sea, regional patterns, decadal changes, and trends are investigated with a view to re-examining the assumption of a general warming trend, as suggested by the observation that the years 1989–1994 represent the mildest winters observed in the past 50, perhaps even 130, years. A regional classification on the basis of an analysis of the patterns of SST anomaly fluctuations suggests eight different regions resembling those identified for the distribution of the North Sea water mass. The oceanic influence on the western side and the continental influence on the eastern side are reflected in the strikingly different behaviour of the western and eastern North Sea SST. Positive anomalies usually start in the Southern Bight and move, with a delay of several weeks, into the central and northern North Sea. None of the regions shows a temperature trend over the last 25 years. However, spectral analysis of long time series and the SST maps clearly indicate climatic fluctuations with a significant peak at a period of about 8 years, a periodicity also found in the North Atlantic Oscillation index (NAO). Salinity at the entrances to the North Sea is high when the NAO indicates an increased zonal atmospheric circulation, whereas the correlation between SST anomaly and NAO is quite low in these areas. By contrast, the correlation is high in the central North Sea. Changes in the air–sea exchange processes and advection of heat and salt from the North Atlantic appear to dominate climatic fluctuations in the North Sea.