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The direct synthesis of highly water-soluble nanoparticles has attracted intensive interest, but systematic size control has not been reported. Here, we developed a general method for synthesizing monodisperse water-soluble iron oxide nanoparticles with nanometer-scale size increments from 4 nm to 13 nm in a single reaction. Precise size control was achieved by continuous growth in an amphiphilic solvent, diethylene glycol (DEG), where the growth step was separated from the nucleation step by sequential addition of a reactant. There was only one reactant in the synthesis and no need for additional capping agents and reducing agents. This study reveals the “living growth” character of iron oxide nanoparticles synthesised in an amphiphilic solvent. The synthetic method shows high reproducibility. The as-prepared iron oxide nanoparticles are extremely water soluble without any surface modification. Surprisingly, the synthesized 9 nm iron oxide nanoparticles exhibit extremely high transversal and longitudinal relaxivities of 425 mM −1 s −1 and 32 mM −1 s −1 respectively, which is among the highest transversal relaxivity in the literature for sub-10 nm spherical nanoparticles. This study will not only shed light on the continuous growth phenomenon of iron oxide nanoparticles in an amphiphilic solvent, but could also stimulate the synthesis and application of iron oxide nanoparticles. The continuous growth method could be further extended to other materials for the controlled synthesis of water-soluble nanoparticles.