Nanocarriers provide a platform to integrate therapy and diagnostics, which is an emerging direction in medical practice. Beyond simply therapeutic functionality, theranostic nanomaterials have been designed to deliver multiple components and imaging agents, facilitating simultaneous and synergistic diagnosis and therapies. In this article, polymeric materials with diverse functionalities and properties for manufacturing theranostic nanomaterials are discussed and compared. We focused on recent advancements in polymeric multifunctional nanomaterials for synergistic theranostics. The drugs and imaging agents were encapsulated within and/or conjugated to the surface of the nanocarriers, according to the fabrication process and carrier type. In parallel with therapy, polymeric multifunctional nanomaterials can be exploited to exhibit distinctive magnetic, electrical, and optical properties for concomitant imaging. This has been accomplished by incorporating various imaging agents, such as fluorescent dyes, biomarkers, quantum dots, metal composites, and magnetic nanoparticles. We discussed theranostic nanomaterial synthesis, carrier fabrication and its applications. By presenting this comprehensive review of the state-of-the-art, we demonstrated that polymeric multifunctional nanomaterials exhibit distinctive advantages and features in theranostics.