The aim of this study was to clarify the effect of universal adhesive (UA) viscosity on the bond strength of resin composite to dentin prepared with Er:YAG laser. Four experimental UAs (SI-1, SI-2, SI-3, and SI-4) were developed by adding 1, 2, 3, and 4 wt/% nanosilica to BeautyBond Xtreme (Shofu), respectively. BeautyBond Xtreme was used as a control (SI-0). The viscosities of experimental UAs were measured using a B-type viscometer. After bovine mandibular anterior teeth were ground with #600 emery paper to obtain the flattened dentin surfaces, the dentin surfaces were cut thinly by irradiating the Er:YAG laser. Specimens were prepared using the respective UA and flowable resin composite and subjected to the microtensile bond strength (µTBS) test. The data from viscosity measurement and the μTBS test were statistically analyzed using the Kruskal–Wallis test. The mean values of viscosity significantly differed among the all experimental groups (p< 0.01). The μTBS of SI-1 and SI-2 was significantly higher than that of SI-0, SI-3, and SI-4 (p< 0.001). The μTBS of SI-0 was significantly lower than that of SI-4 (p< 0.001). The viscosities of the experimental universal adhesives significantly affected their bond strength to laser-cut dentin.