The Dicer nuclease generates small RNAs that regulate diverse biological processes through posttranscriptional gene repression and epigenetic silencing of transcription and recombination. Dicer-deficient cells exhibit impaired differentiation, activity, proliferation, and survival. Dicer inactivation in developing mouse lymphocytes impairs their proliferation and survival and alters Ag receptor gene repertoires for largely undefined reasons. To elucidate functions of Dicer in lymphocyte development and Ag receptor locus transcription and recombination, we analyzed mice with conditional Dicer deletion in thymocytes containing unrearranged or prerearranged TCRβ loci. Expression of either a preassembled functional TCRβ gene (Vβ1(NT)) or the prosurvival BCL2 protein inhibited death and partially rescued proliferative expansion of Dicer-deficient thymocytes. Notably, combined expression of Vβ1(NT) and BCL2 completely rescued proliferative expansion of Dicer-deficient thymocytes and revealed that Dicer promotes survival of cells attempting TCRβ recombination. Finally, inclusion of an endogenous preassembled DJβ complex that enhances Vβ recombination increased death and impaired proliferative expansion of Dicer-deficient thymocytes. These data demonstrate a critical role for Dicer in promoting survival of thymocytes experiencing DNA double-strand breaks (DSBs) during TCRβ recombination. Because DSBs are common and ubiquitous in cells, our findings indicate that impaired cellular survival in response to DSBs should be considered when interpreting Dicer-deficient phenotypes.