Using biocompatible polymers with potential therapeutic activity is an appealing strategy for the development of new functional drug carriers. In this study, we report the synthesis of therapeutic poly(p-coumaric acid) (PCA) from p-coumaric acid, a common plant phenolic acid with multiple bioactivities. The prepared PCA was formulated into nanoparticles (NPs) using the nanoprecipitation method and docetaxel (DTX) was encapsulated to form DTX-loaded PCA NPs (DTX@PCA NPs). Their potential as a nanocarrier for anticancer drug delivery was systematically evaluated. The DTX@PCA NPs not only had a small particle size and good stability, but also exhibited superior in vitro anticancer activity, anti-metastasis ability compared with free drugs, and preferable cellular uptake by tumor cells. In addition, the three-dimensional tumor spheroid assay revealed the effective tumor penetration and anticancer activity of the DTX@PCA NPs. Importantly, the DTX@PCA NPs preferentially accumulated in tumors and prolonged systemic circulation, significantly inhibiting tumor growth in vivo and simultaneously attenuating the side effects of DTX. Interestingly, the blank PCA NPs themselves also exhibited additional tumor suppression activity to some extent with high biosafety, further indicating the significant potential of PCA as a novel self-therapeutic nanocarrier for anticancer drug delivery and enhanced cancer therapy.