Multi-modal imaging-guided cancer photothermal therapy (PTT) with advanced theranostic nanoagents can efficiently improve therapeutic efficacy and reduce treatment side effects. Herein, we have developed a theranostic nanoagent based on indocyanine green (ICG)-loaded polydopamine (PDA)-iron ions coordination nanoparticles (PDA-Fe3+-ICG NPs), which are used for photoacoustic (PA) and magnetic resonance (MR) dual-modal imaging-guided cancer PTT treatments. In this nanoplatform, ICG molecules, the U.S. Food and Drug Administration approved near-infrared (NIR) dye, absorbing on PDA NPs (a melanin-like biopolymer) to significantly increase the NIR optical absorption of PDA NPs nearly 6 times and decreases their fluorescence emission, which can improve the PA contrast ability and promote the photothermal conversion efficiency of PDA NPs. Meanwhile, Fe3+ ions chelated on the PDA NPs act as a T1-weighted MRI contrast agent (r1 = 14 mM-1 s-1). In a mouse 4T1 breast tumor model, PA/MRI dual-modal imaging and highly efficient PTT treatments with low laser density were achieved with remarkable therapeutic efficiency and minimal side effects. This study illustrates that the highly integrated and biocompatible PDA-based NPs can serve as a versatile nanoplatform by loading different imaging molecules and drugs for multi-modal imaging and cancer combination therapy.