Abstract. We measured dissolved methane (CH₄) concentrations, saturations, and fluxes from sea into air and from sediment into water during cruises in March, May, August, October, and December of 2011 in the East China Sea (ECS) and the Yellow Sea (YS). CH₄ concentrations had obvious spatial and seasonal variability due to the complex effects of different water masses and other variables. Maximal CH₄ concentration, sea–air and sediment–water fluxes all occurred during the summer. CH₄ concentration decreased gradually from the coastal area to the open sea, and high levels of CH₄ generally appeared near the Changjiang Estuary and outside the Hangzhou Bay. During early spring and winter, CH₄ had a uniform distribution from the surface to the bottom, but CH₄ concentration increased gradually with depth during other seasons. The subsurface CH₄ maximum occurred at a depth of about 200 m during May, October, and December. The CH₄ level at the bottom was generally higher than at the surface, and this was enhanced during summer due to hypoxia in the bottom waters. Changjiang-diluted water, the Kuroshio Current, and the Taiwan Warm Current Water affected the geographic distribution of CH₄ in the ECS, and these water bodies contributed about 3.45, 2.97, 14.60 mol s⁻¹ of CH₄ during summer and 2.11, 8.58, 5.20 mol s⁻¹ CH₄ during winter, respectively. Sediment was also a significant source of dissolved CH₄ in the ECS, and we estimated the average sediment–water CH₄ flux of the ECS and YS as about 1.02 μmol m⁻² d⁻¹. We also used a box model to calculate the CH₄ budget in the ECS. The results suggested that in situ CH₄ production in the water column was the major source of CH₄, and accounted for 0.21 μmol m⁻³ day⁻¹ during summer and 0.11 μmol m⁻³ day⁻¹ during winter. Air–sea exchange was the major sink of CH₄ in the ECS. We estimated total CH₄ emission from the ECS and YS as about 4.45 x 10⁹ mol during 2011. Our results indicated that the ECS and YS were active areas for CH₄ production and emission.