The development of new contrast agents (CAs) for magnetic resonance imaging (MRI) is of high interest, especially because of the increased concerns of patient safety and quick clearance of clinically used gadolinium and iron oxide-based CAs, respectively. Here, a two-step synthesis of superparamagnetic water-soluble iron platinum (FePt) nanoparticles (NPs) with core sizes between 2 and 8 nm for use as CAs in MRI is reported. First, wet-chemical organometallic NPs are synthesized by thermal decomposition in the presence of stabilizing oleic acid and oleylamine. Second, the hydrophobic NPs are coated with an amphiphilic polymer and transferred into aqueous media. Their magnetization values and relaxation rates exceed those published for CAs already used for clinical application. Their saturation magnetization increases with the core size to approximately 82 A.m(2)/kg(Fe). For 8 nm NPs, the T-2 relaxivity of approximately 221 (mM.s)(-1) is 5 times larger than that for the ferumoxides, and for 6 nm NPs, the T-1 relaxivity of approximately 12 (mM.s)(-1) is slightly higher than that of ultrasmall gadolinium oxide NPs. The 6 nm FePt NPs are identified as excellent CAs for both T-1 and T-2 imaging. Most importantly, because of their coating, significantly low cytotoxicity is achieved. FePt NPs prove to be a promising alternative to gadolinium and iron oxide NPs showing high-quality CA characteristics for both T-1- and T-2-weighted images.