Iron oxide nanoparticles (IO NPs) have become the focus of molecular imaging probes for contrast enhanced magnetic resonance (MR) imaging due to their intrinsic magnetic and biodegradable properties, as well as long blood half-lives and low toxicity. Massive efforts have been made to explore the IO NPs as T2-weighted MR contrast agents, which have high susceptibility to induce a long-range magnetic field that interferes with diagnosis. Thus, the development of IO NPs with potent T1 relaxivity might help in providing an alternative for clinically applied gadolinium chelates. Herein, biomineralized iron oxide-polydopamine hybrid nanodots (IO/PDA-NDs) have been constructed using albumin as the nanoreactors to induce nanoprecipitation and polymerization simultaneously, facilitating T1-weighted contrast-enhancement as well as photothermal therapeutic capability. The IO nanoclusters in IO/PDA-NDs have an r1 relaxivity of 5.79 mM-1 s-1 with a relatively low r2/r1 ratio of 1.71, demonstrating the preferable iron oxide based T1 contrast agents. The high photothermal conversion coefficient and tumor targeting effect of the hybrid nanodots could result in complete tumor ablation efficacy. The biomineralization method provides a promising approach for the integration of tumor diagnosis and treatment to achieve efficient cancer theranostics.