Flow-induced acoustic resonances in piping with closed side branches cause severe structural vibration and fatigue damage of piping and components in power plants. Practical piping systems of power plants often have a steam flow, and moreover, the steam state can be not only dry (i.e., gas single-phase flow with superheated steam) but also wet (i.e., high-quality two-phase flow with mixture of saturated steam and saturated water). Although many researchers have investigated acoustic resonances at side branches, acoustic resonances under a wet steam flow have not yet been clarified since previous studies were mainly conducted under an air flow. Moreover, there have been few previous experiments performed under a steam flow, particularly a wet steam flow. The objective of this study is to investigate acoustic resonances in a closed side branch under a wet steam flow. Experiments on dry and wet steam flows under low pressure and also on an air flow were conducted and the results were compared. Moreover, the applicability of a theoretical equation for the resonance frequency, calculated as the first acoustic mode frequency using the branch piping depth with end correction and the sound speed in dry and wet steam, was evaluated. For our experimental conditions, it was confirmed that the effects of dry steam and air on acoustic resonances were similar. However, higher acoustical damping was confirmed under wet steam than under dry steam, which is considered to be caused by the existing liquid phase (i.e., droplets and/or liquid film). The resonance frequencies under wet steam obtained by the theoretical equation and assuming a saturated sound speed were within ±6% of the measured values, and the critical Strouhal numbers under wet steam were similar to those under dry steam and air when the resonance frequencies were evaluated by the proposed method.