Abstract: Phytophthora palmivora is a destructive oomycete plant pathogen with a wide host range. So far, little is known about the factors governing its infection structure development and pathogenicity. From the culture filtrate of a P. palmivora strain isolated from papaya, we identified a secreted glycoprotein of 15 kDa, designated as Ppal15kDa, using liquid chromatography tandem mass spectrometry. Two gene variants, Ppal15kDaA and Ppal15kDaB were amplified from a P. palmivora papaya isolate. Transient expression of both variants in Nicotiana benthamiana by agroinfiltration enhanced P. palmivora infection. Six Ppal15kDa mutants with diverse mutations were generated via CRISPR/Cas9-mediated gene editing. All mutants were compromised in infectivity on N. benthamiana and papaya. Two mutants with all Ppal15kDa copies mutated almost completely lost pathogenicity. The pathogenicity of the other four containing at least one wild-type copy of Ppal15kDa was compromised at varying levels. The mutants were also affected in development as they produced smaller sporangia, shorter germ tubes, and fewer appressoria. The affected levels in development corresponded to the levels of reduction in pathogenicity, suggesting that Ppal15kDa plays an important role in normal development of P. palmivora infection structures. Consistent with its role in infection structure development and pathogenicity, Ppal15kDa was found to be highly induced during appressorium formation. In addition, Ppal15kDa homologs are broadly present in Phytophthora spp., but none were characterized. Altogether, this study identified a novel component involved in development and pathogenicity of P. palmivora and possibly other Phytophthora spp. known to contain a Ppal15kDa homolog.