Multifunctional nanoprobes with tumor microenvironment response are playing important roles in highly efficient theranostics of cancers. Herein, a kind of theranostic nanoprobe was synthesized by coating manganese dioxide (MnO2) on the surface of black commercial P25 titanium dioxide (b-P25). The resultant nanoprobe (b-P25@MnO2) possessed glutathione (GSH)-responsive magnetic resonance (MR) imaging and enhanced photothermal therapy (PTT). In tumor microenvironments, the excessive GSH was consumed by reacting with MnO2 to generate Mn2+ for GSH-responsive MR imaging, in which the longitudinal relaxation rate of b-P25@MnO2 was up to 30.44 mM-1 s-1, showing excellent cellular and intratumoral MR imaging. Moreover, the prepared b-P25@MnO2 exhibited stable and strong photothermal conversion capability with a high photothermal conversion efficiency of 30.67%, by which the 4T1 tumors disappeared completely, indicating safe and highly efficient PTT performance. The current work developed GSH-responsive b-P25@MnO2 nanoprobes, demonstrated for MR imaging and enhanced PTT in cancers.