We have observed evidence of an incommensurate structural distortion in the ternary chalcogenide Ta2NiSe7. The structure of Ta2NiSe7 is similar to FeNb3Se10 with one tantalum-centered trigonal prismatic chain and two inequivalent chains with octahedral coordination. Anomalies are observed in the resistance and magnetic susceptibility near 52.5 K. X-ray- and electron-diffraction data show the presence of an incommensurate modulation at temperatures below 52.5 K and strong diffuse scattering above 52.5 K. Hysteresis in the resistance and x-ray order parameter suggest that true long-range order of the lattice modulation is never achieved. We interpret the diffuse scattering as resulting from an inhibition of the phase transition caused by strong interactions between the lattice distortion and impurities or defects. Based on the transport data, we interpret the incommensurate structural distortion as being electronic in origin, a charge-density wave.