Zooplankton are the primary link between marine phytoplankton and higher trophic levels. Although there is evidence of a long-term global decline in phytoplankton biomass over the past century in response to climate change, we do not know how zooplankton has responded. This is primarily because of the many ways zooplankton biomass is collected (e.g., different nets and mesh sizes) and measured (e.g., wet weight vs settled volume). Here we use generalised linear mixed models to adjust for these collection and measurement differences and thus standardise 200,000 measurements of zooplankton biomass collected globally from time series and ship-board measurements. In the Australian region, we used zooplankton observations collected in the Integrated Marine Observing System (IMOS) – from the Australian Continuous Plankton Recorder Survey, the National Reference Stations, and a historical data compilation – which makes the Australian EEZ one of the best sampled areas in the Southern Hemisphere. This is the first global analysis of trends in zooplankton biomass globally. The response of zooplankton biomass over time provides insights into how the carrying capacity of the oceans is changing, and implications for biodiversity and carbon sequestration.