Background: Understanding the specific cognitive domains associated with activities of daily living (ADLs) impairment in Lewy body dementia (LBD) may help identify which targeted therapeutic interventions to pursue in future research. This study aimed to investigate the neuropsychological determinants of impairment in ADLs in LBD patients. Methods: We conducted a cross-sectional study of LBD patients referred for a clinical neuropsychological evaluation within the Parkinson's Disease and Movement Disorders Center at Virginia Commonwealth University. The presence or absence of impairment in eight ADLs and neuropsychological test performances were collected and analyzed. Cluster analysis and hierarchical analysis were used to define ADL impairment into mild, moderate, and severe ADL impairment groups. We then compared cognitive performance between the ADL groups. Results: This study included 193 LBD patients. Compared to LBD patients with mild and moderate ADL impairment, those with severe ADL impairment had worse scores in global cognition as measured by the Dementia Rating Scale-2 (DRS-2) (p=0.002), speeded visuospatial processing as measured by the Trail Making Test A (p=0.001), speeded executive functioning as measured by the Trail Making Test B (p=0.006), and psychomotor processing speed (p<0.001). Impairments in driving and self-care were associated with worse performances on Trail Making Test A, Trail Making Test B, and psychomotor processing speed (all p<0.05). Conclusions: In patients with LBD, impaired speeded tasks of visual processing and executive functioning are associated with impaired ADLs, particularly driving and self-care. In order to improve ADLs in LBD, future studies should focus on identifying therapies that improve processing speed performance.