The proper execution of premeiotic S phase is essential to both the maintenance of genomic integrity and accurate chromosome segregation during the meiotic divisions. However, the regulation of premeiotic S phase remains poorly defined in metazoa. Here, we identify the p21(Cip1)/p27(Kip1)/p57(Kip2)-like cyclin-dependent kinase inhibitor (CKI) Dacapo (Dap) as a key regulator of premeiotic S phase and genomic stability during Drosophila oogenesis. In dap(-/-) females, ovarian cysts enter the meiotic cycle with high levels of Cyclin E/cyclin-dependent kinase (Cdk)2 activity and accumulate DNA damage during the premeiotic S phase. High Cyclin E/Cdk2 activity inhibits the accumulation of the replication-licensing factor Doubleparked/Cdt1 (Dup/Cdt1). Accordingly, we find that dap(-/-) ovarian cysts have low levels of Dup/Cdt1. Moreover, mutations in dup/cdt1 dominantly enhance the dap(-/-) DNA damage phenotype. Importantly, the DNA damage observed in dap(-/-) ovarian cysts is independent of the DNA double-strands breaks that initiate meiotic recombination. Together, our data suggest that the CKI Dap promotes the licensing of DNA replication origins for the premeiotic S phase by restricting Cdk activity in the early meiotic cycle. Finally, we report that dap(-/-) ovarian cysts frequently undergo an extramitotic division before meiotic entry, indicating that Dap influences the timing of the mitotic/meiotic transition.