The IUU Fishing Index was launched in early 2019 as a way of benchmarking and ranking countries for their vulnerability to, prevalence of and response to illegal, unreported and unregulated (IUU) fishing. The dedicated IUU Fishing Index website (www.iuufishingindex.net) provides maps to visualize scores by indicator type and responsibility. The 2021 IUU Fishing Index covers all 152 coastal countries of the world, and for each country a score is calculated based on a suite of 40 indicators. These relate to the prevalence of IUU fishing in each country and the country’s vulnerability and response to it, as assessed according to the country’s coastal, flag, port and general state responsibilities. The database underpinning the IUU fishing scores in 2021 contains 5 681 separate data entries, based on both publicly available data and expert opinion, with a high (98%) completion rate across all indicators and countries. The methodology used for the 2021 version of the Index remains the same as for the 2019 version, with one exception, relating to the replacement of one indicator with a more relevant one.