The pure rotational spectrum of diketene has been studied in the millimeter-wave region from similar to 240 to 360 GHz. For the ground vibrational state and five vibrationally excited satellites (v(24), 2v(24), 3v(24), 4v(24), and v(16)), the observed spectrum allowed for the measurement, assignment, and least-squares fitting a total of more than 10 000 distinct rotational transitions. In each case, the transitions were fit to single-state, complete or near-complete sextic centrifugally distorted rotor models to near experimental error limits using Kisiel's ASFIT. Additionally, we obtained less satisfactory least-squares fits to single-state centrifugally distorted rotor models for three additional vibrational states: v(24) + v(16), v(23), and 5v(24). The structure of diketene was optimized at the CCSD(T)/ANO1 level, and the vibration-rotation interaction (alpha(i)) values for each normal mode were determined with a CCSD(T)/ANO1 VPT2 anharmonic frequency calculation. These alpha(i) values were helpful in identifying the previously unreported v(16) and v(23) fundamental States. We obtained a single-crystal X-ray structure of diketene at -173 degrees C. The bond distances are increased in precision by more than an order of magnitude compared to those in the 1958 X-ray crystal structure. The improved accuracy of the crystal structure geometry resolves the discrepancy between previous computational and experimental structures. The rotational transition frequencies provided herein should be useful for a millimeter-wave or terahertz search for diketene in the interstellar medium.