Aims: We report on the millimeter-wave rotational spectrum of protonated sulfur dioxide, HOSO. Methods: Ten rotational transitions between 186 and 347 GHz have been measured to high accuracy in a negative glow discharge. Results: The present measurements improve the accuracy of the previously reported centimeter-wave spectrum by two orders of magnitude, allowing a frequency calculation of the principal transitions to about 4 km s-1 in equivalent radial velocity near 650 GHz, or one linewidth in hot cores and corinos. Conclusions: Owing to the high abundance of sulfur-bearing molecules in many galactic molecular sources, the HOSO ion is an excellent candidate for detection, especially in hot cores and corinos in which SO2 and several positive ions are prominent.
