Development of multifunctional theranostic nanoplatforms for diagnosis and therapy of cancer still remains a great challenge. In this work, we report the use of hyaluronic acid-modified Fe3O4@Au core/shell nanostars (Fe3O4@Au-HA NSs) for tri-mode magnetic resonance (MR), computed tomography (CT), and thermal imaging and photothermal therapy of tumors. In our approach, hydrothermally synthesized Fe3O4@Ag nanoparticles (NPs) were used as seeds to form Fe3O4@Au NSs in the growth solution. Further sequential modification of polyethyleneimine (PEI) and HA affords the NSs with excellent colloidal stability, good biocompatibility, and targeting specificity to CD44 receptor-overexpressing cancer cells. With the Fe3O4 core NPs and the star-shaped Au shell, the formed Fe3O4@Au-HA NSs are able to be used as a nanoprobe for efficient MR and CT imaging of cancer cells in vitro and the xenografted tumor model in vivo. Likewise, the NIR absorption property enables the developed Fe3O4@Au-HA NSs to be used as a nanoprobe for thermal imaging of tumors in vivo and photothermal ablation of cancer cells in vitro and xenografted tumor model in vivo. This study demonstrates a unique multifunctional theranostic nanoplatform for multi-mode imaging and photothermal therapy of tumors, which may find applications in theranostics of different types of cancer.