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Japan has 16 native species of the genus Hosta Tratt. (Asparagaceae). A recent study on Hosta based on field surveys and molecular phylogenetic analyses resulted in the discovery of six unknown taxa in Kochi Prefecture, Shikoku Island, southwestern Japan. We aimed to identify these unknown taxa. Therefore, we constructed a finely resolved phylogeny for 320 Hosta samples collected from the Honshu, Shikoku, and Kyushu Islands using multiplex inter-simple sequence repeat genotyping by sequencing (MIG-seq). Based on this phylogenetic analysis and related morphological observations, we describe five new species, H. longipedicellata sp. nov., H. minazukiflora sp. nov., H. polyneuronoides sp. nov., H. samukazemontana sp. nov., and H. takiminazukiflora sp. nov. and one new subspecies, H. takiminazukiflora subsp. grandis subsp. nov. In addition, we propose two new status assignments, H. tardiva subsp. densinervia comb. and stat. nov. and H. scabrinervia stat. nov. We also propose classifying H. kikutii var. tosana as a species, H. tosana. Further studies that combine MIG-seq with careful morphological observations are needed for Hosta plants on all Japanese islands, which may result in the discovery of even more undescribed species.