Methanesulfonic acid (MSA), derived from the oxidation of dimethylsulfide (DMS), has a significant impact on biogenic sulfur cycle and climate. Gaseous MSA (MSAg) has been often ignored in previous studies due to its quick conversion to particulate MSA (MSAp) and low concentrations. MSAg, MSAp, and nss-SO42- were observed simultaneously for the first time with high-time-resolution (1 h) in the Southern Ocean (SO). The mean MSAg level reached up to 3.3 ± 1.6 pptv, ranging from ∼24.5 pptv in the SO, contributing to 31% ± 3% to the total MSA (MSAT). A reduction of the MSA to nss-SO42- ratios by about 30% was obtained when MSAg was not accounted for in the calculation, indicating that MSAg was very important in the assessment of the biogenic sulfur contributions in the atmosphere. Mass ratios of MSA to nss-SO42- increased first and then decreased with the temperature from -10 to 5 °C, with a maximum value at the temperature of -3 °C. Positive correlations between MSAg to MSAT ratios and temperature were presented, when the temperature was higher than 5 °C. This study highlights the importance of MSAg for understanding the atmospheric DMS oxidation mechanism and extends the knowledge of MSA formation in the marine atmosphere.