The 2,4-pentadiynyl radical, H2C-C=C-C=CH, was detected in a pulsed-discharge supersonic molecular beam by Fourier transform microwave spectroscopy. Four successive rotational transitions, N = 2-1 through N = 5-4 in the K-a = 0 ladder were observed between 8 and 22 GHz. A total of 48 hyperfine components were measured and assigned with an uncertainty of 5 kHz. Seven spectroscopic constants, including rotational and centrifugal distortion constants, the spin-rotation constant, and hyperfine coupling constants for the methylenic and acetylenic protons were determined to high accuracy.