The room-temperature, aqueous-phase synthesis of iron-oxide nanoparticles (IO NPs) with glutathione (GSH) is reported. The simple, one-step reduction involves GSH as a capping agent and tetrakis(hydroxymethyl)phosphonium chloride (THPC) as the reducing agent; GSH is an anti-oxidant that is abundant in the human body while THPC is commonly used in the synthesis of noble-metal clusters. Due to their low magnetization and good water-dispersibility, the resulting GSH-IO NPs, which are 3.72 ± 0.12 nm in diameter, exhibit a low r2 relaxivity (8.28 mm−1s−1) and r2/r1 ratio (2.28)—both of which are critical for T1 contrast agents. This, together with the excellent biocompatibility, makes these NPs an ideal candidate to be a T1 contrast agent. Its capability in cellular imaging is illustrated by the high signal intensity in the T1-weighted magnetic resonance imaging (MRI) of treated HeLa cells. Surprisingly, the GSH-IO NPs escape ingestion by the hepatic reticuloendothelial system, enabling strong vascular enhancement at the internal carotid artery and superior sagittal sinus, where detection of the thrombus is critical for diagnosing a stroke. Moreover, serial T1- and T2-weighted time-dependent MR images are resolved for a rat's kidneys, unveiling detailed cortical-medullary anatomy and renal physiological functions. The newly developed GSH-IO NPs thus open a new dimension in efforts towards high-performance, long-circulating MRI contrast agents that have biotargeting potential.