Cancer is one of the major threats to human health. For the treatment of cancer, photothermal therapy (PTT) is a highly effective and minimal invasive therapeutic approach. Indocyanine green (ICG), approved by the U.S Food and Drug Administration (FDA) for clinical use, is widely utilized as an optical imaging agent. It can be used not only for fluorescence imaging but also for a photothermal therapeutic agent. Because it is capable of converting absorbed light energy into heat that induces local hyperthermia. Moreover, ICG has strong light absorption at a biological transparent near-infrared region, so it can take an advantage of deeper tissue penetration. In this respect, ICG has been considered as an ideal platform for cancer theranostics. However, ICG has certain limitations including poor photo stability and non-specific binding to proteins (e.g. albumin and lipoprotein). To overcome these limitations, we designed polymeric nanoparticles wherein ICG molecules are effectively loaded through the formation of hydrophobic complexes with biocompatible cations. Among various types of cations, Fe3+ ions were shown to form stable hydrophobic complexes with ICG due to strong charge interaction between anionic sulfonate groups of ICG and cationic Fe3+ with a ratio of 3 to 1. The nanoparticle was fabricated through physical nano assembly of all biocompatible constituents: a hydrophobic complex of ICG, Fe3+ and pluronic F127 (a biocompatible block copolymer surfactant). Formulated ICG-Fe nanoparticles (ICG-Fe NPs) have spherical morphology with the average diameter of 16.9 ± 2.5 nm. ICG-Fe NPs were capable of retaining their structural integrity under physiological circumstances, and their fluorescence, a competitive photophysical pathway for transduction of absorbed energy other than heat generation, was significantly quenched. This means that the photothermal effect of the ICG-Fe NPs can be maximized in comparison to aqueous solution of ICG (free ICG). For toxicity evaluation of ICG-Fe NPs, no obvious decease in cell viability was observed even treated with high concentration of ICG-Fe NPs. Upon systemic administration into a tumor bearing mouse, ICG-Fe NPs showed longer circulation time and higher delivery efficiency toward a tumor region compared to those of free ICG. Furthermore, after an intravenous injection of ICG-Fe NPs followed by photothermal therapy, tumor growth was suppressed and treatments induced toxicity was observed. Since ICG-Fe NPs produce photoacoustic signals, they also can be used as photoacoustic imaging agents. All the results reveal that ICG-Fe NPs hold a promising candidate for biomedical applications as a theranostic agent.