INTRODUCTION: Lewy body dementia (LBD) shares genetic risk factors with Alzheimer's disease (AD), including apolipoprotein E (APOE), but is distinguishable at the genome-wide level. Polygenic risk scores (PRS) may therefore improve diagnostic classification. METHODS: We assessed diagnostic classification using AD-PRS excluding APOE (AD-PRS RESULTS: Together AD-PRS DISCUSSION: Aβ deposition in LBD was associated with APOE, while MCI+/AD was also associated with AD-PRS beyond APOE. AD-PRS explains phenotypic variance not captured by APOE or p-tau181. HIGHLIGHTS: We investigated Alzheimer's disease (AD) polygenic risk score (PRS), apolipoprotein E (APOE), and plasma phosphorylated tau 181 (p-tau181) to classify AD and Lewy body dementia (LBD). AD-PRS with APOE achieved similar classification accuracy to p-tau181. AD-PRS without APOE significantly contributed to discriminating AD from LBD. Amyloid beta positivity in LBD was associated with APOE but not AD-PRS without APOE or p-tau181. Combining AD-PRS, APOE, and p-tau181 improved diagnostic classification accuracy.