Boron subphthalocyanines have been explored as functional materials in a wide range of organic electronic applications including organic light-emitting diodes and organic solar cells. They have only, however, been studied as small molecules; there have been no reports of boron subphthalocyanine polymer(s) in the literature. Herein, the synthesis of the first boron subphthalocyanine-containing polymer, poly(4-methylstyrene)-co-poly(phenoxy boron subphthalocyanine), is reported along with its basic physical characterization/properties. The synthesis of the boron subphthalocyanine polymer was only possible via a postpolymerization functionalization approach, where the boron subphthalocyanine moiety is appended to the side chain of a styrene-based prepolymer. The three-step transformation begins with the nitroxide-mediated copolymerization of 4-methylstyrene and 4-acetoxystyrene to form poly(4-methylstyrene)-co-poly(4-acetoxystyrene). The acetoxy groups are subsequently deacetylated to afford hydroxyl groups prior to phenoxylation with bromo boron subphthalocyanine to afford the title polymer(s).