The synthesis and structural characterization of poly(trimethylene-1,1-dicarboxylic acid) (CH2-CH2-C-(COOH)(2))(n), a carbon-chain polymer bearing two carboxylic acids on every third carbon alongside the backbone, are described for the first time. Solid-state properties are investigated by a variety of techniques (WAXS, FT-IR, CP-MAS, and TGA), in particular with respect to the semicrystalline character of the polymer and its affinity toward water. The observed experimental features are rationalized on the basis of the ability for the macromolecular chains to associate via the pendant carboxylic acids and of the high global symmetry obtained for this polyacid through the geminal placement of the side substituents.