The COVID-19 pandemic has resulted in unparalleled global impacts on human mobility. In the ocean, ship-based activities are thought to have decreased due to severe restrictions and changes in goods consumption, but little is known of the patterns of change, which sectors are most affected, in which regions, and for how long. Here, we map global change of marine traffic during the COVID-19 pandemic and assess its temporal variability at a fine-scale in one of the most affected regions, the Mediterranean Sea. Nearly 44.3% of the global ocean and 77.5% of national jurisdictions showed a decrease in traffic density during April 2020, when strictest confinement measures took place, showing a clear disruption in comparison with previous trends and future projections. Decreases mainly occurred in coastal areas and were more marked and longer lasting in sectors other than cargo and tanker shipping. Our results provide guidance for large-scale monitoring of the progress and potential effects of COVID-19, or other global shocks, on the blue economy and ocean health.