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The wide range of physical and chemical properties of polyurethanes make them one of the most popular polymers for industrial applications. Polyurethanes are synthesized using isocyanates, polyols, surfactants, additives, and catalysts. Polyurethane foams can be rigid or flexible and find vast applications in the automotive, construction, furniture, and medical industries. Despite the numerous applications of polyurethanes, their high flammability is a major concern for their safe use in many applications. In addition, current research seeks renewable sources such as vegetable oils and other biomass for polyurethane synthesis. In our research, highly flame-retardant polyurethane foams using bio-derived polyol were prepared and characterized. Sunflower oil was used in the synthesis of a bio-based polyol as an alternative to petroleum-based polyols. Epoxidation followed by ring-opening reactions were carried out to synthesize the sunflower oil polyol whose formation was confirmed with other tests. Expandable graphite (EG) and dimethyl methylphosphonate (DMMP) which are non-halogenated flame retardants were used in varying concentrations for the preparation of bio-based rigid polyurethane foams. The effects of these flame retardants on the physicomechanical properties and flame retardancy of the sunflower-based foams were studied. Mechanical and thermogravimetric analysis showed that the foams had a high compressive strength along with high thermal stability. The closed-cell contents of the foams were over 90% with a uniform distribution of cell size. The burning test revealed a significant effect of the flame-retardants on the flammability of the foams. And with the addition of EG, the burning weight loss time was reduced from 80 s to 4 s and that of DMMP from 70 s to 3 s. Our research suggests that sunflower oil could be a potential candidate for the polyurethane industries and the use of non-halogenated DMMP or EG can serve as green and effective flame-retardants in bio-based polyurethane foams.