The aim of the research has been to investigate the change of dynamic behavior of a Kaplan turbine runner when submerged in still water. More specifically, the effects of the added mass on the modes of vibration have been quantified. For that, the modes of vibration of the runner in vacuum, in air and in water have been simulated with a coupled Acoustic-Structural modal analysis and, from their comparison, the reduction of the natural frequencies and the possible changes of the mode shapes have been determined. The results show that the typically assumed invariance of all the mode shapes of a structure when submerged in a dense fluid is not fully accomplished in this particular geometry. And regards to the frequency reduction ratios, they are found to be similar for the first six modes with an average value of about 37%.