Highly cross-linked and monodispersed poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) submicrospheres were synthesized by one-step polycondesation. The spheres are enriched in N, P, and S atoms, and phenolic hydroxyl groups, and exhibit good water dispersion. These properties facilitate initial growth of gold nanoseeds on the PZS surface and subsequent growth of gold nanoshells. This facile fabrication method can also be used for the preparation of silver and gold–silver bimetallic nanoshells with tunable thickness by varying the mass ratio of metal salt to PZS. The core@shell structures were investigated by transmission electron microscopy, energy-dispersive X-ray spectrometry, selected-area electron diffraction and X-ray diffraction analysis. PZS@gold nanoshells showed very strong surface plasmon resonance absorption in the near infrared region (700–1100 nm), where blood and biological tissues are nearly transparent. Furthermore, PZS@gold nanoshells effectively induced photothermal conversion under laser irradiation of 808 nm and rapidly killed tumor cells, suggesting potential application in photothermal imaging and therapies using NIR lasers.