The H2CCC- ion is studied by autodetachment spectroscopy in a coaxial laser-ion beam spectrometer. Sharp resonances at photon energies near the photodetachment threshold energy are ascribed to a (2)A(1) <-- B-2(1) transition followed by autodetachment of the dipole-bound state (DBS). Some 2500 rotational transitions are assigned and the band origin is determined to be 14 284.420(5) cm(-1). The observation of individual rotational lines allowed determination of the rotational spectroscopic constants as A=9.651 53(4) cm(-1) and (B + C)/2=0.346 461(3) cm(-1) for the DBS as well as the spin-rotation coupling constant epsilon(aa)=2.17(6)x10(-3) cm(-1). Based on an electron affinity of 14 469+/-64 cm(-1), the binding energy of the electron in the DBS is 170+/-50 cm(-1). Anomalous rotational line positions are found in the vicinity of K-a=7-10 in the DBS and have been attributed to the centrifugal distortion couplings caused by mixing with the CCC out-of-plane bending mode (nu(6)) and the CCC in-plane bending mode (nu(9)). The linewidths provide information about autodetachment rates that make it possible to obtain electron detachment dynamics for individual rotational states.