Introduction: The effects of air-polishing on exposed root dentin surfaces are largely unknown, as there are only few studies which show heterogeneous results. Thus, this study was to investigate roughness changes of exposed dentin surfaces after air-polishing and the influence of subsequent polishing with cup and paste. Methods: Totally 54 teeth with exposed root dentin surfaces were treated using a split-mouth design by either air-polishing with erythritol and a rubber cup with polishing paste on the test side, or rubber cup and paste alone. Teeth were finally cleaned using a sonic tooth brush. Impressions were taken at relevant time points and replicated using epoxy resin. The resulting casts were profilometrically analysed to obtain the average surface roughness (sR_a) and maximum peak-to-valley height (sR_z), which are given as the mean ± standard deviation in μm. Results: After air-polishing, in comparison to the baseline, there was a slight but significant increase in sR_a (0.168 ± 0.143, p < 0.001), but sR_z did not change (−0.471 ± 4.857, p = 0.936). Subsequent polishing with cup and paste and cleaning with a sonic toothbrush did not reduce the surface roughness (sonic toothbrush-air-polishing, sR_a −0.044 ± 0.081, p = 0.218; sR_z −0.551 ± 3.563, p = 0.903). Conclusion: The use of erythritol led to a slight increase in the roughness of the dentin surface, which was not reduced by polishing with a cup and paste. Polishing paste did not seem to conceal surface irregularities.