•The Mediterranean is a semi-enclosed sea subject to high mercury pollution.•Monitoring temporal trends of mercury concentration is compulsory.•Dolphins integrate long-term, large-scale pollution variations in oceanic waters.•A decline in Mediterranean Hg levels is shown to occur from 1990 to 2009. The Mediterranean is a semi-enclosed sea subject to high mercury (Hg) pollution from both natural and anthropogenic sources. With the objective of discerning temporal changes in marine Hg pollution in the oceanic waters of the northwestern Mediterranean Sea, we analysed liver and kidney from striped dolphins (Stenella coeruleoalba) collected during 2007–2009 and compared them with previous results from a similar sample from 1990–1993. The effect of body length and sex on tissue Hg concentrations was investigated to ensure an unbiased comparison between the periods. The Hg concentrations did not show significant sex-related differences in any tissue or period but were correlated positively with body length. Using body length as a covariate, Hg concentrations in liver and kidney were higher in 1990–1993 than in 2007–2009. This result suggests that measures to reduce emissions in Western European countries have been effective in reducing mercury pollution in Mediterranean open waters.