This paper provides data on non-native fungus-like organisms (Oomycota, kingdom Stramenopila), their diversity, and distribution in Lithuania and is an addition to a previously published checklist of alien true fungi (Chytridiomycota, Ascomycota, and Basidiomycota; kingdom Fungi). All available published data were summarized, and the analysis showed that approximately 25% of recorded Lithuania stramenopilous fungus-like organism species are alien. The compiled list represented herein includes 54 species of alien terrestrial and aquatic stramenopilous fungus-like organisms (Oomycota). Most alien Oomycota are obligate pathogens of terrestrial plants. Aquatic organisms are represented by one alien dangerous crayfish pathogen, Aphanomyces astaci, which is currently noted as extinct in Lithuania. The taxonomy of the listed alien species of Oomycota has been revised, and the species names have been adjusted to reflect current nomenclature changes. The taxonomical and ecological analysis demonstrated the prevalence of terrestrial parasitic Peronosporales reported in Lithuania Oomycota and the highest diversity of alien species belonging to the Peronospora genus. The chronology of appearance, distribution within the country, and current status of these non-native organisms were assessed, and some of the most economically or ecologically important harmful invasive species were discussed.