Here, we report a tannic acid-Fe coordination complex coating that confers magnetic resonance imaging (MRI) theranostic properties to inert nanomaterials . Boron nitride nanosheets (BNS), which lack magnetic field and light responsiveness, were used as a model nonfunctional nanomaterial . Among various catechol derivatives tested (i.e., dopamine , 3,4-dihydroxyphenylacetic acid, gallic acid, and tannic acid), a coating of tannic acid-Fe coordination complex provided the highest magnetic field relaxivity and near infrared (NIR) laser light responsiveness . An in vitro study showed that KB tumor cells treated with tannic acid-Fe coordination complex adsorbed on BNS (TA-Fe/BNS) exhibited higher T1-weighted magnetic resonance contrast compared with plain BNS, and BNS coated with tannic acid or Fe alone . NIR irradiation at 808 nm caused a significant increase in KB tumor cell death after treatment with TA-Fe/BNS compared with other treatments . In vivo MRI imaging revealed tumor accumulation of intravenously administered TA-Fe/BNS . Guided by MRI information, application of focused laser irradiation onto tumor tissues resulted in complete tumor ablation . These results support the potential of TA-Fe/BNS for MRI theranostics . Moreover, this study suggests the wide applicability of TA-Fe noncovalent coating as biocompatible and facile tool for converting nonfunctional early-generation nanomaterials into functional new nanomaterials, opening new opportunities for their use in translational biomedical applications such as MRI theranostics.