Theranostic platforms that combine therapeutic and imaging modalities have received increasing interest. The development of theranostic nanovectors that can release therapeutic agents at pathological tissues in an on-demand manner and provide instantaneous feedback through non-invasive imaging techniques is of urgent need. Herein, a new magnetic resonance imaging (MRI) contrast agent, gadolinium oxide (Gd2O3), and an anticancer drug, doxorubicin (DOX), were co-loaded into mesoporous silica nanoparticles (MSNs) with the formation of hybrid Gd2O3@MSN-DOX nanoparticles. The hybrid nanoparticles were further coated with pH-responsive polyelectrolytes that underwent a charge reversal process at acidic pH. Upon entering into cells by folic acid (FA) receptor-mediated endocytosis, the mildly acidic pH within endolysosomes triggered the disassociation of the absorbed polyelectrolytes on the surfaces of the Gd2O3@MSN-DOX nanoparticles and thus actuated the DOX release, thereby exerting an anti-cancer effect. More importantly, the confinement of paramagnetic Gd2O3 within MSNs led to a remarkable increase of MRI relaxivity (r1 = 9.14 mM-1 s-1 vs. 3.68 mM-1 s-1 of the clinically applied MRI contrast agent), likely due to the increased tumbling time and coordination number of water molecules. This work provides a feasible strategy to fabricate theranostic nanovectors with controlled release behavior triggered by mildly acidic pH and high-performance MR imaging capability.