Differential object marking (DOM) in Spanish refers to the overt morphological marking of certain direct objects. Specifically, this a-marking of direct objects is driven by animacy and usually precedes human objects. Other features such as specificity and definiteness matter to a lesser extent. This direct object a-marking has also been attested in Brazilian Portuguese (BP), but with more restrictions than in Spanish. Thus, BP is typically not considered a DOM language. This article discusses the acquisition of DOM in second language (L2) Spanish among BP speakers. Seventy-four adult Brazilians with various levels of L2 proficiency completed three experimental tasks: elicited production, acceptability, and productive vocabulary knowledge, which measures productive lexical knowledge and was designed specifically to assess this language pair. We analyzed our data to uncover the knowledge that BP-speaking L2 learners of Spanish possess of DOM and to determine whether their knowledge of DOM can be predicted by their L2 productive vocabulary knowledge. Results indicate that the learners acquire the animacy-driven nature of Spanish DOM, both in their productive and receptive grammatical knowledge, as their productive vocabulary knowledge increases. The L2 learners present different acquisitional outcomes yet those with deeper productive vocabulary knowledge show a stronger animacy-driven distribution of DOM in Spanish.