Accurate diagnosis of diseases located in deep tissues is always challenging. The "always-on" probe often leads to false-positive signals due to nonspecific interaction of nanoprobes. Thus, stimuli-responsive nanoprobes are highly desirable, which, however, require complicated surface modification so as to achieve trigger-induced signal changes. Here pH-triggered switchable magnetic resonance imaging (MRI) nanoprobes were constructed by coordination-driven self-assembly of monodispersed iron oxide nanoparticles (MIONPs) with simple amino acid derivatives, which displayed typical T 2 -weighted MRI features, yet, were turned into T 1 -weighted MRI under slightly acidic conditions at the tumor site. The dynamic assembly and disassembly properties of MIONPs afford T 2 / T 1 switchable contrast imaging, enabling selective "turn-on" signals at the tumor site with high specificity.