Protected areas (PAs) are essential for biodiversity conservation but are threatened by cropland expansion. Recent studies have only reported global cropland expansion in large PAs between 1990 and 2005. However, the amount of cropland expansion in global PAs (including relatively small PAs) since the 2000s is unclear. Using 30-m cropland maps, we find that the cropland expansion in PAs accelerated dramatically from 2000 to 2019, compared with that of global croplands. The areal expansion was mainly in large PAs, less-strict PAs and Afrotropical PAs, which also matches the higher species extinction risks. Such PAs appear to be less effective due to greater threats, such as higher background cropland expansion rate. Notably, some PAs with the highest conservation levels failed to prevent cropland expansion. This new picture of cropland dynamics in PAs illustrates that cropland expansion is an ongoing intractable global conservation challenge that will impinge on the aspirations of the post-2020 global biodiversity framework.