Aims. Morphological skills in Williams syndrome (WS) are a controversial issue, particularly cross-linguistically. Methods. We assessed pluralization patterns of nouns, inflection of verbs in the past, and gender assignment, inflection, and agreement within the noun phrase in a sample of Spanish-speaking children with WS compared to typically developing (TD) children matched on mental age (VA-TD) and on chronological age (CA-TD) age. Results. Children with WS attribute grammatical gender correctly in a production task, but they have problems with inferring the referent’s sex from the gender of the noun in a comprehension task. Additionally, they correctly pluralize nouns and properly inflect regular verbs, but they have problems with irregular verbs. Our findings suggest that they have mastered the productive rules, but they perform like younger children regarding irregular items.