Litchi downy blight, caused by the plant pathogenic oomycete Peronophythora litchii, is one of the most devastating diseases on litchi and resulted in huge economic losses.  Autophagy plays an essential role in the development and pathogenicity of the filamentous fungi.  However, the function of autophagy in oomycetes remain elusive.  Here, an autophagy-related protein Atg3 homolog PlAtg3 was identified and characterized in P. litchii.  The absence of PlATG3 through the CRISPR/Cas9 gene replacement strategy compromised vegetative growth and sexual/asexual development.  Cytological analyses revealed that the deletion of PlATG3 impaired autophagosome formation with monodansylcadaverine (MDC) staining and significantly disrupted zoospore release due to defects of sporangial cleavage with FM4-64 staining.  Atg8 is considered to be an autophagy marker protein in various species.  Western blot analysis indicated that PlAtg3 is involved in degradation of PlAtg8-PE.  Interestingly, PlAtg3 was unable to interact with PlAtg8 in yeast two hybrid (Y2H) assays, possibly due to the absence of the Atg8 family interacting motif (AIM) in PlAtg3.  Furthermore, pathogenicity assays revealed that the deletion of PlATG3 considerably reduced the virulence of P. litchii.  Taken together, our data reveal that PlAtg3 plays an important role in radial growth, asexual/sexual development, sporangial cleavage and zoospore release, autophagosome formation, and pathogenicity in P. litchii.  This study contributes to a better understanding of the pathogenicity mechanisms of P. litchii and provides insights for the development of more effective strategies to control oomycete diseases.