Modification of epoxy resins with organophosphorus compounds, either as reactive co-reactants or additives, is the key to achieving non-flammable advanced epoxy materials. Herein, through a straightforward and cost-effective approach, epoxy thermosets were prepared by simply mixing a new phosphorus flame retardant (13.5 wt% phosphorus) with a bifunctional bisphenol-A based epoxy polymer, followed by thermal curing in the presence of an aromatic aminic hardener. It was proved that a very low content of phosphorus led to composites exhibiting remarkable improved flame retardancy. 1 wt% phosphorus was enough to increase the limiting oxygen index value to about 30%. Furthermore, the peak of the heat release rate was reduced up to 45%, depending on the content of the flame retardant additive introduced into the epoxy matrix. UL-94 V-0 materials were obtained when 2 and 3 wt% phosphorus were added into the epoxy matrix. Thermogravimetric data showed that the incorporation of the flame retardant additive significantly increased the char yield and thermal stability of the gradually forming phosphorus-rich carbonaceous layer at elevated temperatures.