We evaluated the effectiveness of an online training on palliative care knowledge and self-efficacy among staff working with people with intellectual and developmental disabilities (IDD) using a one-group pretest-posttest design. Staff from four nonprofit residential and day services organizations in a U.S. Midwestern state participated. Among 132 staff who completed a baseline assessment, a 2-hour online training, and a posttest, 98 staff completed a 1-month follow-up survey. Palliative care knowledge was assessed before and after the training, and palliative care self-efficacy, at baseline and 1-month follow-up. We used linear regression to identify the factors that influence the effect of the training on main outcomes. Overall palliative care knowledge and self-efficacy significantly improved while higher education and longer work tenure enhanced training effectiveness.