Inorganic nanoparticle-based T1 contrast agents with high longitudinal relaxivity (r1) and low r2/r1 ratio have attracted great interest in recent years. However, the r1 relaxivity of inorganic nanoparticles reported to date is relatively low. In this work, 2.3 ± 0.1 nm paramagnetic gadolinium hydrated carbonate nanoparticles (GHC-1) with a high r1 relaxivity of 34.8 mM-1 s-1 and low r2/r1 ratio of 1.17 are synthesized using a one-pot hydrothermal process. The r1 of GHC-1 is 9.4 times higher than that of Gd-DTPA at 0.55 T. The synthetic procedure is simple, cost effective, and easy to scale up. The nanoparticles have a small core size, an amorphous phase, and are well-coated by poly(acrylic acid). Due to the hydrophilic polymer coating, the particles are highly dispersible and stable in aqueous solution. No significant cellular or in vivo toxicity are observed for the nanoparticles, which guarantees the in vivo application of this material. Finally, we apply the nanoparticles to in vivo magnetic resonance imaging and study the biodistribution in organs. This study reveals GHC-1 as a potential candidate for a T1 contrast agent with extraordinary ability to enhance MR images.