Little is known about whether manipulatives can support children's inferences of the mathematical structure of word problems. The objective was to test the effects of using manipulatives during problem solving on students' understanding of the additive relationships in word problems. Third and fifth graders (N = 45) solved one-step addition and subtraction word problems that described either a mathematical action or relation. Children in each grade were randomly assigned to either a manipulatives or a paper-and-pencil condition. Problem structure understanding was assessed by the degree to which students' strategies and verbal justifications reflected the quantitative relationships described in the problem text. Performance of the fifth graders was not impacted by the use of manipulatives, but in the third grade, strategy performance with manipulatives was superior to performance without manipulatives on Relation problems. The degree to which the third graders' justifications were aligned with corresponding problem structure was positively impacted by manipulatives regardless of problem type.