A structurally novel hyperbranched halogen-free poly(phosphoester) (hbPPE) is proposed as a flame retardant in poly(ester)s and epoxy resins. hb polymeric flame retardants combine several advantages that make them an extraordinary approach for future flame retardants. hbPPE was synthesized by olefin metathesis polymerization according to a straightforward two-step protocol. The impact of hbPPE on pyrolysis, flammability (reaction-to-small-flame), and fire behavior under forced flaming conditions (cone calorimeter) was investigated for a model substance representing poly(ester)s, i.e. ethyl 4-hydroxybenzoate, and an epoxy resin of bisphenol A diglycidyl ether cured with isophorone diamine. The flame retardancy performance and mechanisms are discussed and compared to a commercial bisphenol A bis(diphenyl phosphate) (BDP). Both hbPPE and BDP combined gas-phase and condensed-phase activity; hbPPE is the more efficient flame retardant, and is proposed to be efficient in a greater variety of polymeric matrices. The hydrolysis of hbPPE is suggested to produce phosphorous acids, which, when available at the right temperatures, enhance the charring of the polymer in the condensed phase. The better fire protection behavior of the hbPPE is due not only to its higher phosphorus content, but also to the higher efficiency of the phosphorus it contains.