Theranostic polymers have emerged as a versatile platform in cancer nanomedicine, integrating therapeutic and imaging functionalities to overcome challenges in oncology. Featuring diverse architectures such as linear polymers, dendrimers, star-like polymers, and bottle-brush polymers, these systems enable tumor-targeted drug delivery, real-time imaging, and controlled release. Recent advances in stimuli-responsive designs and biomimetic strategies have improved their specificity, stability, and adaptability, outperforming conventional nanocarriers. This review summarizes the design, synthesis, and biomedical applications of theranostic polymers, focusing on their potential to address tumor heterogeneity and biological barriers. The challenges of biocompatibility, immunogenicity, and clinical translation are discussed, with a perspective toward future developments in precision medicine and imaging-guided cancer therapy.