|Abstract||This study explored young adult siblings' use of confirming behaviors as a moderator of sibling rivalry and relational outcomes in the sibling relationship (i.e., closeness and satisfaction). Participants included 329 young adults who completed online questionnaires concerning their perceptions of their sibling relationship, including their parents' treatment of them and their siblings, sibling confirmation, sibling challenge, and closeness and satisfaction with the sibling relationship. Bivariate correlations supported the hypothesized negative associations among perceptions of parents' differential treatment (i.e., an indicator of rivalry) and sibling confirmation, challenge, closeness, and satisfaction, as well as the positive associations among sibling confirmation, challenge, closeness, and satisfaction. Multivariate tests were conducted separately for participants' perceptions of differential parental treatment that favored them and/or their siblings, and the results of these tests provided partial support for the hypothesis that sibling confirmation would mitigate the negative effects of a sibling rivalry on sibling closeness and satisfaction. Specifically, moderate to high levels of sibling confirmation from the target sibling helped mitigate the negative effect of parents' differential treatment of the target sibling on the participant's closeness and satisfaction with the target sibling. Among the more important implications of this study is the finding that confirmation moderates the negative effect of rivalry on the sibling relationship, but only when young adults report that their sibling perceives that s/he is the recipient of their parents' differential treatment.