Significance Increasing evidence demonstrates that small RNAs can serve as trafficking effectors to mediate bidirectional transkingdom RNA interference (RNAi) in interacting organisms, including plant–pathogenic fungi systems. Previous findings demonstrated that plants can send microRNAs (miRNAs) to fungal pathogen Verticillium dahliae to trigger antifungal RNAi. Here we report that V. dahliae is able to secret an effector to the plant nucleus to interfere with the nuclear export of AGO1–miRNA complexes, leading to an inhibition in antifungal RNAi and increased virulence in plants. Thus, we reveal an antagonistic mechanism that can be exploited by fungal pathogens to counteract antifungal RNAi immunity via manipulation of plant small RNA function.