The Motivation Assessment Scale is an aid for hypothesis-driven functional analysis. This study presents its Spanish cross-cultural validation while examining psychometric attributes not yet explored. The study sample comprised 80 primary caregivers of children with autism. Acceptability, scaling assumptions, internal consistency, factor structure, inter-assessor reliability and agreement, and known-group validity analyses were performed. Scaling assumptions, internal consistency (Cronbach alpha of 0.75) and factor structure were satisfactory other than for the Escape domain which demonstrated low internal consistency (0.65), inadequate scaling assumptions (multitrait analysis, 50% success rate) and did not constitute a separate factor. Caregivers' agreement for the primary function reached 73.9% and known group-validity hypotheses across behavior topographies were partially met. The clinical appropriateness of the scale is discussed.