The modern biodiversity crisis reflects global extinctions and local introductions. Human activities have dramatically altered rates and scales of processes that regulate biodiversity at local scales 1–7. Reconciling the threat of global biodiversity loss 2, 4, 6–9 with recent evidence of stability at fine spatial scales 10,11 is a major challenge and requires a nuanced approach to biodiversity change that integrates ecological understanding. With a new dataset of 471 diversity time series spanning from 1962 to 2015 from marine coastal ecosystems, we tested (1) whether biodiversity changed at local scales in recent decades, and (2) whether we can ignore ecological context (e.g., proximate human impacts, trophic level, spatial scale) and still make informative inferences regarding local change. We detected a predominant signal of increasing species richness in coastal systems since 1962 in our dataset, though net species loss was associated with localized effects of anthropogenic impacts. Our geographically extensive dataset is unlikely to be a random sample of marine coastal habitats; impacted sites (3% of our time series) were underrepresented relative to their global presence. These local-scale patterns do not contradict the prospect of accelerating global extinctions 2,4,6–9 but are consistent with local species loss in areas with direct human impacts and increases in diversity due to invasions and range expansions in lower impact areas. Attempts to detect and understand local biodiversity trends are incomplete without information on local human activities and ecological context. •Recent temporal change in marine biodiversity was context dependent•Local net gain of species was the prevailing signal in our dataset•Biodiversity change depended on human impacts, community structure, and spatial scale•Long-term monitoring sites are unlikely to be a random sample of the global ocean Elahi et al. use a global synthesis of marine time series to demonstrate that local biodiversity change in recent decades depended on human impacts, community structure, and spatial scale. A synthetic understanding of local biodiversity trends is incomplete without information on local ecological conditions.