A limonene-based flame-retardant compound, a method of making a flame-retardant polymer, and an article of manufacture comprising a material that includes a limonene-based flame-retardant compound are provided. In an embodiment, the method includes forming a limonene-based derivative; forming a phosphorus-based flame-retardant molecule; reacting the limonene-based derivative with the phosphorus-based flame-retardant molecule to form a limonene-based flame-retardant compound; and forming a flame-retardant polymer from the limonene-based flame-retardant compound. In some embodiments, the limonene-based flame-retardant compound has variable functionality including vinyl, epoxide, methylene bridges, and thioethers.