The nitroxide-containing nucleoside Çm is reported as the first rigid spin label for paramagnetic modification of RNA by solid-phase synthesis. The spin label is well accommodated in several RNA secondary structures as judged by its minor effect on the thermodynamic stability of hairpin and duplex RNA. Electron paramagnetic resonance (EPR) spectroscopic characterization of mono-, bi-, and trimolecular RNA structures shows that Çm will be applicable for advanced EPR studies to elucidate structural and dynamic aspects of folded RNA.