Fungal small RNAs suppress plant immunity by hijacking host RNA interference (RNAi) pathways. Botrytis cinerea, a fungal pathogen causing gray mold disease, produces small RNAs (sRNAs) that silence host genes involved in immunity. These Bc-sRNAs bind to Arabidopsis Argonaute 1 (AGO1) and selectively silence host immunity genes. The Arabidopsis ago1 mutant shows reduced susceptibility to B. cinerea, and the B. cinerea dcl1 dcl2 double mutant, which cannot produce these sRNAs, shows reduced pathogenicity. This demonstrates that B. cinerea uses sRNA effectors to suppress host immunity and achieve infection, representing a naturally occurring cross-kingdom RNAi mechanism. B. cinerea infects many vegetable and fruit crops, causing significant economic losses. It is a useful model for studying aggressive fungal pathogens. Previously, pathogen effectors were thought to be proteins, but B. cinerea sRNAs act as effectors to suppress host immunity. sRNAs induce gene silencing by binding to AGO proteins and directing the RNA-induced silencing complex (RISC) to complementary sequences. sRNAs from both plant and animal hosts regulate host-microbial interactions. Although sRNAs are present in fungi and oomycetes, their role in host-pathogen interactions was unclear. To explore B. cinerea sRNAs in pathogenicity, sRNA libraries were profiled from infected plants and controls. A total of 832 sRNAs were identified that matched the B. cinerea genome but not plant genomes. Among them, 27 had predicted miRNA-like precursor structures. These sRNAs targeted host genes in both Arabidopsis and tomato. For example, Bc-siR3.2 targeted mitogen-activated protein kinases (MAPKs), which are likely involved in plant immunity. Bc-sRNAs were shown to suppress host genes during infection, as evidenced by reduced expression of target genes in infected plants. To confirm this, coexpression assays in Nicotiana benthamiana showed that Bc-sRNAs reduced expression of target genes, while Arabidopsis miR395 did not. Silencing was abolished when target genes had mutated versions of the relevant Bc-sRNA target sites. Transgenic Arabidopsis plants expressing Bc-sRNAs showed enhanced susceptibility to B. cinerea, indicating that these sRNAs play a positive role in pathogenicity. The targets of Bc-sRNAs, such as MAPKs and WAK, are involved in plant immunity. Mutant plants with disrupted target genes showed enhanced susceptibility to B. cinerea. The Bc-siR3.2 target, MAPKKK4, was also suppressed upon infection. These results suggest that Bc-sRNAs silence plant genes to suppress host immunity during early infectionFungal small RNAs suppress plant immunity by hijacking host RNA interference (RNAi) pathways. Botrytis cinerea, a fungal pathogen causing gray mold disease, produces small RNAs (sRNAs) that silence host genes involved in immunity. These Bc-sRNAs bind to Arabidopsis Argonaute 1 (AGO1) and selectively silence host immunity genes. The Arabidopsis ago1 mutant shows reduced susceptibility to B. cinerea, and the B. cinerea dcl1 dcl2 double mutant, which cannot produce these sRNAs, shows reduced pathogenicity. This demonstrates that B. cinerea uses sRNA effectors to suppress host immunity and achieve infection, representing a naturally occurring cross-kingdom RNAi mechanism. B. cinerea infects many vegetable and fruit crops, causing significant economic losses. It is a useful model for studying aggressive fungal pathogens. Previously, pathogen effectors were thought to be proteins, but B. cinerea sRNAs act as effectors to suppress host immunity. sRNAs induce gene silencing by binding to AGO proteins and directing the RNA-induced silencing complex (RISC) to complementary sequences. sRNAs from both plant and animal hosts regulate host-microbial interactions. Although sRNAs are present in fungi and oomycetes, their role in host-pathogen interactions was unclear. To explore B. cinerea sRNAs in pathogenicity, sRNA libraries were profiled from infected plants and controls. A total of 832 sRNAs were identified that matched the B. cinerea genome but not plant genomes. Among them, 27 had predicted miRNA-like precursor structures. These sRNAs targeted host genes in both Arabidopsis and tomato. For example, Bc-siR3.2 targeted mitogen-activated protein kinases (MAPKs), which are likely involved in plant immunity. Bc-sRNAs were shown to suppress host genes during infection, as evidenced by reduced expression of target genes in infected plants. To confirm this, coexpression assays in Nicotiana benthamiana showed that Bc-sRNAs reduced expression of target genes, while Arabidopsis miR395 did not. Silencing was abolished when target genes had mutated versions of the relevant Bc-sRNA target sites. Transgenic Arabidopsis plants expressing Bc-sRNAs showed enhanced susceptibility to B. cinerea, indicating that these sRNAs play a positive role in pathogenicity. The targets of Bc-sRNAs, such as MAPKs and WAK, are involved in plant immunity. Mutant plants with disrupted target genes showed enhanced susceptibility to B. cinerea. The Bc-siR3.2 target, MAPKKK4, was also suppressed upon infection. These results suggest that Bc-sRNAs silence plant genes to suppress host immunity during early infection