Group contingencies have been used successfully to modify a variety of behaviors for children with diverse characteristics across multiple settings. However, these interventions have not been applied to increase social interactions (SI) between typically developing children and those with multiple or severe disabilities (MSD). Furthermore, little research has been conducted to examine whether differential outcomes are associated with the type of reward used (known or mystery). The purpose of this study was to examine the differential effects of known versus mystery rewards on the SI of preschool children with and without MSD within an independent group contingency. The findings indicated that although there were no differences in levels of SI between reward types, both were superior to the baseline condition and were viewed as socially valid by classroom teachers and naive raters. In addition, this study was conducted with high methodological quality exceeding that of previous group contingency research conducted in preschool settings and of other studies examining the differential effects of known and mystery rewards. The results provide meaningful information regarding practices that support children with MSD and add to the group contingency literature.