peer reviewed ; During the European heatwave of 2018 that led to record-breaking temperatures in many countries across northern and central Europe, average seawater temperature in July was 2.5 °C higher than the mean from 2004 to 2017 for same month in the Belgian coastal zone (BCZ) (Southern Bight of the North Sea). The mean dissolved CH4 concentration in surface waters in July 2018 (338 nmol L−1) was three times higher than in July 2016 (110 nmol L−1), and an extremely high dissolved CH4 concentration in surface waters (1607 nmol L−1) was observed at one near-shore station. The high dissolved CH4 concentrations in surface waters in the BCZ in July 2018 seemed to be due to a combination of enhancement of methanogenesis and of release of CH4 from gassy sediments, both most likely related to warmer conditions. The emission of CH4 from the BCZ to the atmosphere was higher in 2018 compared to 2016 by 57% in July (599 versus 382 μmol m−2 d−1) and by 37% at annual scale (221 versus 161 μmol m−2 d−1). The European heatwave of 2018 seems to have led to a major increase of CH4 concentrations in surface waters and CH4 emissions to the atmosphere in the BCZ.