Background: Reliable and valid screening instruments for depression and anxiety are needed for adults with intellectual disabilities. Methods: Internal consistency (n = 198), inter-rater reliability (n = 41), test–retest reliability (n = 37) and criterion validity (n = 43) were studied in adults aged between 18 and 49 years. Internal consistency was also studied in a sample with epilepsy (n = 98). Results: Internal consistencies of the Dutch ADAMS total scale and subscales were satisfactory to good (α = 0.76–0.92), as well as in the subgroup with epilepsy (α = 0.74–0.88). Inter-rater reliability and test–retest reliability were fair to excellent for the total scale (ICC’s: 0.57–0.84) and subscales (ICC’s: 0.43–0.86). The criterion validity of the Dutch ADAMS Depressive Mood subscale was good with a sensitivity of 88% (95% CI: 53%–98%) and a specificity of 80% (95% CI: 64%–90%). Conclusions: Our study shows that the Dutch ADAMS is a reliable and valid instrument for adults aged between 18 and 49 years with intellectual disabilities (and comorbid epilepsy).