In the context of joint decision-making, we investigated whether preschoolers alter the informativeness of their justifications depending on the common ground that they share with their partner. Pairs of 3- and 5-year-olds (N = 146) were introduced to a novel animal with unique characteristics (e.g., eating rocks). In the common ground condition, the children learned about the animal together. In the one-expert condition, one learned about it, the other was naive. In the two-experts condition, children learned about it separately. Later, the pairs had to decide together on 3 items that the novel animal might need. Both age groups referred to the unique characteristics of the animal in their justifications more in the 2 conditions without common ground than in the common ground condition. Thus, preschoolers begin to use common ground flexibly in their justifications and reason-giving in peer interactions.