The encapsulation of nitroxide radicals within ultrashort (ca. 50 nm) single-walled carbon nanotubes (US-tubes) is achieved. Tempo- and Iodo-Tempo@US-tubes are characterized by thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Electron paramagnetic resonance (EPR) spectra display characteristic signals due to the detection of the spin probes within the US-tubes. Longitudinal proton relaxivities (r1) of both nitroxide@US-tubes samples are 7 to 13 times greater than the free nitroxide radicals in solution, giving relaxivities comparable to the clinical contrast agent (CA) Magnevist. In addition, transverse proton relaxivities (r2) show unprecedented proton relaxation enhancement in comparison to any other reported nitroxide radical-based system or the clinically approved T2 CA, Resovist, under the same conditions. T2-weighted magnetic resonance imaging (MRI) phantom images show that the encapsulation of nitroxide radicals within the US-tubes produces good contrast enhancement due to their high r2 relaxivities. The nitroxide radicals@US-tube agents are a new promising class of spin probes for MRI and electronic paramagnetic resonance imaging (EPRI) labeling, tracking, and diagnosis.