Using a molecular jet Fourier transform spectrometer, the microwave spectrum of E-3-penten-2-one, also called methyl propenyl ketone, was recorded in a range from 2.0 to 26.5 GHz. Two conformers, antiperiplanar (ap) and synperiplanar (sp), could be identified. Complicated splitting patterns arising due to the internal rotation of the acetyl methyl and propenyl methyl groups could be resolved, and all measured rotational transitions were fitted to measurement accuracy. The barrier heights of the acetyl methyl group are 434.149(37) cm−1 and 358.076(26) cm−1 for the ap and sp conformers, respectively. For the propenyl methyl group, the barrier is 581.903(45) cm−1 for the ap conformer and 595.271(71) cm−1 for the sp conformer. By comparing these results to those of other ketones categorized in a class system connecting the internal rotation of the acetyl methyl group to the structures of the molecules, we extended the categorization with a new class for α,β-unsaturated ketones.