A flame-retardant vanillin-derived cross-linker, a process for forming a flame-retardant polymer, and an article of manufacture comprising a material that contains the flame-retardant vanillin-derived cross-linker are disclosed. The flame-retardant vanillin-derived cross-linker can be synthesized from vanillin obtained from a bio-based source, and can have at least one phosphoryl or phosphonyl moiety with phenyl, allyl, epoxide, propylene carbonate, or thioether substituents. The process for forming the flame-retardant polymer can include reacting a diol vanillin derivative and a flame-retardant phosphorus-based molecule to form the flame-retardant vanillin-derived cross-linker, and binding the flame-retardant vanillin-derived cross-linker to a polymer. The material in the article of manufacture can be flame-retardant, and contain flame-retardant vanillin-derived cross-linkers. Examples of materials that can be in the article of manufacture can include resins, plastics, adhesives, polymers, etc.