A key skill in collaborative problem-solving is to communicate and evaluate reasons for proposals to arrive at the decision benefiting all group members. Although it is well-documented that collaborative contexts facilitate young children’s reasoning, less is known about whether competition with other groups contributes to children’s collaborative reasoning. We investigated whether between-group competition facilitates children’s within-group collaborative reasoning, regarding their production of reasons and their use of transacts, communicative acts that operate on one another’s proposals and reasoning. We presented 5- and 7- year-old peer dyads with two collaborative problem-solving tasks (decorating a zoo and a dollhouse). In one task, children competed against another group (the competitive condition); whereas in the other task, they did not (non-competitive condition). Our results suggest that children’s sensitivity to group competition as reflected in their reasoning changed depending on the task. When they decorated a house, they produced more transacts in the competitive condition than in the non-competitive condition; whereas when they decorated a zoo, this pattern was reversed. Thus, our results highlight that group competition did not influence children’s collaborative reasoning consistently across different contexts.