The intertwining between the life cycle of plants and their pathogens made the plant circadian clock an integral constituent of the plant immune system. Reciprocally, pathogens were also found to perturb the expression pattern of certain clock genes. However, how pathogens influence clock components remains largely unknown. Here we show that an oomycete effector HaRxL10 directly targets Arabidopsis central clock component CCA1 HIKING EXPEDITION (CHE) to manipulate its function. HaRxL10 stabilises CHE by disrupting E3 ligase ZEITLUPE-mediated CHE protein degradation. Surprisingly, the accumulation of CHE does not enhance but rather suppresses CHE function, inhibiting its binding to the downstream gene promoter. HaRxL10 triggers reprogramming of the transcriptome including expression of genes related to circadian oscillations. Moreover, HaRxL10 hijacks CHE to repress plant immunity and manipulate physiological processes, including hypocotyl growth and flowering. Taken together, our study discovers the first plant pathogen effector that directly targets a plant circadian clock component and elucidates the underlying molecular mechanism.