The majority of sulfur-containing molecules detected in the interstellar medium (ISM) are analogs of oxygen-containing compounds. Propynal was detected in the ISM in 1988, hence propynethial, its sulfur derivative, is a good target for an ISM search. Our aim is to measure the rotational spectrum of propynethial and use those measurements to search for this species in the ISM. To date, measurements of the rotational spectra of propynethial have been limited to a small number or transitions below 52GHz. The extrapolation of the prediction to lines in the milimeter-wave domain is inaccurate and does not provide data to permit an unambiguous detection. The rotational spectrum was re-investigated up to 630GHz. Using the new prediction lines of propynethial, as well as the related propynal, a variety of astronomical sources were searched, including star-forming regions and dark clouds. A total of 3288 transitions were newly assigned and fit together with those from previous studies, reaching quantum numbers up to J=107 and K_a_=24. Watson's symmetric top Hamiltonian in the I^r^ representation was used for the analysis, because the molecule is very close to the prolate limit. The search for propynethial resulted in a non-detection; upper limits to the column density were derived in each source.