E. coli RecQ protein is a multifunctional helicase with homologs that include the S. cerevisiae Sgs1 helicase and the H. sapiens Wrn and Blm helicases. Here we show that RecQ helicase unwinds a covalently closed double-stranded DNA (dsDNA) substrate and that this activity specifically stimulates E. coli topoisomerase III (Topo III) to fully catenate dsDNA molecules. We propose that these proteins functionally interact and that their shared activity is responsible for control of DNA recombination. RecQ helicase has a comparable effect on the Topo III homolog of S. cerevisiae, consistent with other RecQ and Topo III homologs acting together in a similar capacity. These findings highlight a novel, conserved activity that offers insight into the function of the other RecQ-like helicases.