Break-induced replication (BIR) is a pathway of homology-directed repair that repairs one-ended DNA breaks, such as those formed at broken replication forks or uncapped telomeres. In contrast to conventional S phase DNA synthesis, BIR proceeds by a migrating D-loop and results in conservative synthesis of the nascent strands. DNA polymerase delta (Pol δ) initiates BIR; however, it is not known whether synthesis of the invading strand switches to a different polymerase or how the complementary strand is synthesized. By using alleles of the replicative DNA polymerases that are permissive for ribonucleotide incorporation, thus generating a signature of their action in the genome that can be identified by hydrolytic end sequencing, we show that Pol δ replicates both the invading and the complementary strand during BIR. In support of this conclusion, we show that depletion of Pol δ from cells reduces BIR, whereas depletion of Pol ε has no effect. Display omitted •Pol δ synthesizes both strands during BIR•Pol α-primase and DNA ligase I are required for BIR•DNA synthesis during BIR is independent of Pol ε•HydEn-seq technology is applied to DNA repair synthesis Donnianni et al. elucidate DNA polymerase usage during break-induced replication (BIR). In contrast to conventional S phase DNA replication, in which DNA Pol ε synthesizes the leading strand and Pols α and δ synthesize the lagging strand, the authors show that Pol δ synthesizes both strands during BIR.