During DNA repair by homologous recombination (HR), DNA synthesis copies information from a template DNA molecule. Multiple DNA polymerases have been implicated in repair-specific DNA synthesis(1-3), but it has remained unclear whether a DNA helicase is involved in this reaction. A good candidate DNA helicase is Pif1, an evolutionarily conserved helicase in Saccharomyces cerevisiae important for break-induced replication (BIR)(4) as well as HR-dependent telomere maintenance in the absence of telomerase(5) found in 10-15% of all cancers(6). Pif1 has a role in DNA synthesis across hard-to-replicate sites(7,8) and in lagging-strand synthesis with polymerase delta (Pol delta)(9-11). Here we provide evidence that Pif1 stimulates DNA synthesis during BIR and crossover recombination. The initial steps of BIR occur normally in Pif1-deficient cells, but Pol delta recruitment and DNA synthesis are decreased, resulting in premature resolution of DNA intermediates into half-crossovers. Purified Pif1 protein strongly stimulates Pol delta-mediated DNA synthesis from a D-loop made by the Rad51 recombinase. Notably, Pif1 liberates the newly synthesized strand to prevent the accumulation of topological constraint and to facilitate extensive DNA synthesis via the establishment of a migrating D-loop structure. Our results uncover a novel function of Pif1 and provide insights into the mechanism of HR.