Near-infrared (NIR)-to-NIR upconverting NaY(Gd)F4:Tm3+,Yb3+ paramagnetic nanoparticles (NPs) are efficiently detected by NIR imaging techniques. As they contain Gd3+ ions, they also provide efficient “positive” contrast in magnetic resonance imaging (MRI). Water-dispersible small (≈25 nm, “S-”) and ultrasmall (<5 nm diam., “US-”) NaY(Gd)F4:Tm3+,Yb3+ NPs are synthesized by thermal decomposition and capped with citrate. The surface of citrate-coated US-NPs shows sodium depletion and high Gd elemental ratios, as confirmed by a comparative X-ray photoelectron spectroscopy (XPS)/neutron absorption analysis study. US-NaGd0.745F4:Tm0.005,Yb0.25 NPs have hydrodynamic diameters close to that measured by TEM, with the lowest relaxometric ratios (r2/r1 = 1.18) reported for NaGdF4 nanoparticle suspensions (r1 = 3.37 mM−1 s−1 at 1.4 T and 37 °C). Strong relaxivity peaks in the range of 20 (0.47 T) - 300 MHz (7.05 T) are revealed in nuclear magnetic resonance dispersion profiles, with high r2/r1 ratios at increasing field strengths for S-NPs. This indicates the superiority of US-NPs over S-NPs for achieving high positive contrast at clinical MRI field strengths. I.-v. injected citrate-coated US-NPs evidence long blood retention times (>90 min) in mice. Biodistribution studies (48 h, 8 d) show elimination through the reticuloendothelial and urinary systems, similarly to other citrate-capped US-NP systems. In summary, upconverting NaY(Gd)F4:Tm3+,Yb3+ nanoparticles have promising luminescent, relaxometric and blood-retention properties for dual MRI/optical imaging.