Question We evaluated the effect of disturbance on the productivity-diversity relationship in a long-term monitoring study. We asked whether the same productivity-diversity relationship applies to 12 years of pre-fire (undisturbed) conditions and to eight years of post-fire succession studied in the same plots. Location Bugac, Kiskunsag, Central Hungary. Methods We studied 20 permanent plots for 20 years in grassland patches of a forest-steppe vegetation complex, 12 years before (2000-2011), and eight years after (2012-2019) a severe wildfire. The cover values of each vascular plant species were visually estimated each year. We used total cover as a proxy for productivity and species richness as a measure of diversity. We assessed changes in the productivity-diversity relationship before and after the disturbance event. Temporal changes of the pre-fire and post-fire relationship were analysed separately by generalized estimation equations in the R environment. Results In the pre-fire period, we found a positive linear productivity-diversity relationship, and no time effect. However, in the post-fire period, we found a unimodal relationship, which changed gradually from year to year. The disturbance event moved the vegetation out of a stable state, increased the range of both productivity and diversity, and resulted in a decreasing linear component of the relationship after the fire. Our results provide a striking example of the influence of succession on the shape of the productivity-diversity relationship. Conclusions Disturbance may create considerable and long-lasting changes in the productivity-diversity relationship of formerly stable communities. The changing shape of the productivity-diversity relationship over time after disturbance suggests that the evaluation of broad-scale productivity-diversity relationships should control for disturbance history.