The C5N radical has been detected for the first time by using a pulsed-discharge-nozzle Fourier transform microwave spectrometer. The radical was generated by a discharge in a mixture of HC5N and HC3N diluted in Ar and subsequently cooled to a few kelvins in a supersonic free jet expansion. The rotational transitions of C5N have been observed with resolved fine and hyperfine structures in the range from 5 to 17 GHz. The ground state of C5N was determined to have 2Σ symmetry. The present laboratory observation have allowed us to perform a deep radio astronomical search for C5N in interstellar molecular clouds.