Despite how important it is to assess executive functioning in persons with Intellectual Disability (ID), instruments adapted and validated for this population are scarce. This study’s primary goal was to find evidence for the validity of the ID version of the Tower of London (TOLDXtm) test in persons with mild (IDMi) and moderate (IDMo) levels of ID with Down Syndrome (DS). A multicenter study was carried out. Subjects (n = 63, ≥ 39 years old) had DS with mild (n = 39) or moderate ID (n = 24) with no minor neurocognitive disorder or Alzheimer’s disease. Assessment protocol: TOLDXtm for ID, Kaufman Brief Intelligence Test Second Edition (K-BIT II), Cambridge Examination for Mental Disorders of Older People with Down’s Syndrome and Others with Intellectual Disabilities (CAMDEX-DS), Weigl’s Color-Form Sorting Test (WCFST), Barcelona Test for Intellectual Disability (BT-ID), and the Behavior Rating Inventory of Executive Function (BRIEF-P). The internal consistency (IDMi and IDMo), factor structure of the different subscales, and relationship between TOLDXtm subscales and other cognitive measures (BT-ID, WCFST, and BRIEF-P) were analyzed. A normative data table with ID population quartiles is provided. TOLDXtm for ID showed a robust one factor structure and coherentassociations with other, related neuropsychological instruments. Significant differences between IDMi and IDMo on movement-related variables like Correct (Corr; p = .002) and Moves (Mov; p = .042) were observed, along with good internal consistency values, Corr (α = .75), Mov (α = .52). Regarding internal consistency, no between-groups differences were observed (all p-value > 0.05). The TOLDXtm for ID is thus an instrument, supported by good validity evidence, to evaluate problem-solving and planning in ID. It distinguishes between individuals with mild and moderate ID, and is highly associated with other measures of executive functioning.