Interstellar cyanoallene (CH2CCHCN) has been detected with the 100 m Green Bank Telescope (GBT) toward the dark Taurus molecular cloud (TMC-1) by means of the 414-313, 404-303, 515-414, and 505-404 rotational transitions at 20.2, 20.6, 25.2, and 25.7 GHz, respectively. The hyperfine structure (hfs) of the 414-313 transition is resolved; the remaining transitions have hfs components that are blended but are inferred owing to line width excesses atypical of TMC-1. An analysis of these four rotational transitions yields an average column density NT=2.0(5) ×1012 cm-2 at an assumed rotational temperature of 4 K. The cyanoallene isomer methylcyanoacetylene (CH3CCCN) has previously been observed in TMC-1 and found to have a rotational temperature of ~4 K by other investigators. Using the GBT, the K=0 and K=1 structure of the JK=5K-4K transition of methylcyanoacetylene was resolved toward TMC-1 and a total column density of NT=4.5(4)×1011 cm-2 was obtained for the 4 K rotational temperature. Thus, cyanoallene appears to be more abundant than methylcyanoacetylene toward TMC-1 by a factor of ~4.5. The implications of this result are discussed in terms of relative bonding energy differences and possible differences in formation chemistry.