The entomopathogen Bacillus thuringiensis species harbours numerous plasmids essentially studied for their involvement in pathogenicity, as Cry-plasmids. The life cycle of B. thuringiensis in the insect host is regulated by the sequential activation of quorum sensing systems to kill, survive and sporulate. In this study, we characterize a new quorum sensing system belonging to the Rap-Phr family. The Rap8-Phr8 system is borne by the pHT8_1 plasmid, a small cryptic plasmid from the B. thuringiensis var. kurstaki HD73 strain. Our results demonstrate that the Rap8 protein inhibits sporulation and biofilm formation through the Spo0A pathway. The Rap8 activity is inhibited by the mature Phr8 heptapeptide YAHGKDI. The key residues specific for the Rap phosphatase activity are conserved in Rap8 suggesting a common mode of action. Interestingly, we show that the Rap8-Phr8 system is specifically required for regulating sporulation of B. thuringiensis in insect larvae. This system may allow the bacteria to exert a tight control of the sporulation process in the host cadaver for optimizing the multiplication, the survival and the dissemination of the bacteria. Thus, our results suggest that pHT8_1 provides advantages for adaptation and evolution of B. thuringiensis in its ecological niche.