Background: Due to its centrality in the conceptualization of intellectual disability, reliable and valid measurement of adaptive behaviour is important to both research and clinical practice. The manual of the Vineland Adaptive Behavior Scales, recently released in its third edition, provides limited reliability information obtained from a sample composed primarily of typically developing individuals. The goal of this study was to evaluate the concordance of the Vineland-3 with the Vineland-II in a sample more similar in ability level to those in which the Vineland is commonly used. Methods: Both editions of the Vineland Comprehensive Parent Interviews were conducted with a convenience sample of 106 parents/caregivers of individuals with neurodevelopmental disability, participating at two neurodevelopmental disorder research clinics. Administrations were up to 7 days apart, but most (90%) were simultaneous. The concordance correlation coefficient (CCC) [95% CI] and mean differences [95% CI] were calculated for domain standard scores and subdomain v -scale scores. Results: Domain-level CCC ranged from 0.78 [0.70, 0.84] for Communication to 0.86 [0.76, 0.92] for Motor. Subdomain CCC ranged from 0.71 [0.62, 0.78] for Receptive Language to 0.91 [0.85, 0.95] for Fine Motor. Vineland-3 scores were lower than Vineland-II scores; 77% of participants had lower Adaptive Behavior Composite scores on the Vineland-3 than on the Vineland-II. For the subdomains, the magnitude of this difference depended upon the level of adaptive behaviour. For Communication, the domain with the lowest CCC, the mean difference ranged from −13.70 [−8.03, −19.35] for a Vineland-II score or 85 to a difference of −19.18 [−12.28, −26.87] for a Vineland-II score of 40. Discussion: Among individuals with intellectual and developmental disabilities, the Vineland-3 produces lower scores than the Vineland-II, and these clinically significant differences tend to be larger for individuals with lower levels of ability. Thus, care must be taken in interpreting scores from the Vineland-3 relative to those obtained from the previous edition.