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By means of Fabry-Perot FT microwave spectroscopy, the rotational spectrum of protonated sulfur dioxide in two distinct isomeric forms, a \textit{cis} and a \textit{trans} geometry, is reported. Searches for both isomers were based on theoretical structures obtained at the CCSD(T)/cc-pwCVQZ level of theory corrected for zero-point vibrational effects at the CCSD(T)/cc-pV(T+$d$)Z level. At a similarly high level of theory, the \textit{cis} isomer is calculated to be the global minimum on the potential energy surface, but the \textit{trans} isomer is predicted to lie only a few kcal/mol higher in energy. The identification of \textit{cis} HOSO$+$ has been confirmed by detection of rotational lines of DOSO$+$ and HO$\mathrm{<mrow\; data-mjx-texclass="ORD">34</mrow>}$SO$+$ at precisely the expected frequency shifts. Because sulfur dioxide is a well known interstellar and extragalactic species, and because it possesses a large proton affinity, HOSO$+$ is a excellent candidate for radioastronomical detection.