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Research on mathematics achievement by children with Williams syndrome (WS) has been very limited. We describe the math achievement of 72 9-year-olds with WS, compare their math and reading achievement, and explore concurrent predictors of math achievement using the Wechsler Individual Achievement Test-III (WIAT-III) to measure achievement and the Differential Ability Scales-II (DAS-II) to measure cognitive abilities. For both Numerical Operations and Math Problem Solving, mean standard scores (SSs) were in the mild disability range with a full range from severe disability to average ability. Bayesian robust estimation indicated decisive evidence that average reading performance was higher than average math performance. Bayesian multiple linear regression models with informative priors accounted for 60% of the variance in Numerical Operations SS with decisive evidence supporting an effect of Working Memory SS and strong evidence for Nonverbal Reasoning SS and Spatial SS and 71% of the variance in Math Problem Solving SS with decisive evidence supporting effects of Working Memory and Nonverbal Reasoning SS, strong evidence for Verbal SS, and substantial evidence for Spatial SS. These predictors are consistent with those for typically developing individuals, supporting consideration of interventions that have been highly effective for children with math difficulties in the general population., (© 2024. The Author(s).)