Development of novel activable dual-mode T1/T2-weighted magnetic resonance (MR) contrast agents with the same composition for dynamic precision imaging of tumors has been a challenging task. Here, we demonstrated a strategy to prepare clustered Fe3O4 nanoparticles (NPs) with redox-responsiveness to tumor microenvironment to achieve switchable T2/T1-weighted dual-mode MR imaging applications. In this study, we first synthesized ultrasmall Fe3O4 NPs with an average size of 3.3 nm and an r1 relaxivity of 4.3 mM-1 s-1, and then cross-linked the single Fe3O4 NPs using cystamine dihydrochloride (Cys) to form clustered Fe3O4/Cys NPs. The Fe3O4 nanoclusters (NCs) possess desirable colloidal stability, cytocompatibility, high r2 relaxivity (26.4 mM-1 s-1), and improved cellular uptake efficiency. Importantly, with the redox-responsiveness of the disulfide bond of Cys, the Fe3O4 NCs can be dissociated to form single particles under a reducing condition, hence displaying a switchable T2/T1-weighted MR imaging property that can be utilized for dynamic precision imaging of cancer cells in vitro and a subcutaneous tumor model in vivo due to the reductive intracellular environment of cancer cells and the tumor microenvironment. With the tumor reductive microenvironment-mediated switching of T2 to T1 MR effect and the ultrasmall size of the single particles that can pass through the kidney filter, the developed Fe3O4 NCs may be used as a promising switchable T2/T1 dual-mode MR contrast agent for precision imaging of different biosystems.