The aim of the study was to investigate the effect of the relative humidity of the ambient air on the dentin bonding efficiency of four self-etching primer adhesives. Three commercially available self-etching primer adhesives, AQ Bond, Fluorobond, One-Up Bond F, and one experimental all-in-one adhesive, AC-Bond, were selected. Peripheral plane dentin surfaces of human molars were prepared with wet SiC paper. Eight specimens were prepared for each of the adhesives and each of the following six conditions prior to application of the bonding agents and a hybrid-type resin composite material: air dried, wet, or stored for 1 h at 33%, 50%, 75%, and 100% relative humidity (RH). Shear bond strength (SBS) was determined after 24 h of storage in water at 37°C. None of the adhesives showed statistically significant differences in SBS between the experimental conditions. AC Bond, AQ Bond, and Fluorobond were equally effective with an average SBS of 19.0 MPa, whereas One-Up Bond F showed a significantly lower mean SBS of 11.5 MPa. The self-etching primer adhesives tested are unaffected by the degree of dentin wetness or ambient air humidity exposure prior to application. Unless considered necessary for other clinical reasons, use of rubber-dam is not compulsory.