The objective of this laboratory study was to assess the effect of immediate dentin sealing (IDS) on the fracture strength of teeth restored with ultrathin occlusal veneers made of a polymer-infiltrated ceramic network (PICN - Vita Enamic) with different fissure/cusp thicknesses. Forty third molars were identically prepared in the dentin and allocated into four groups (n = 10) according to the fissure/cusp thickness of the occlusal veneers (0.3/0.6 mm or 0.6/0.9 mm) and the application or not of the IDS technique. Ten sound third molars were used as the control group. IDS was performed using a universal adhesive system (Single Bond Universal). The prepared teeth were scanned (Cerec software, version 4.1), and the occlusal veneers were shaped following the occlusal anatomy of a right second molar. Blocks of PICN were milled in the unit inLab MC XL. Following luting with Single Bond Universal and RelyX Ultimate, the fracture strength of the specimens was obtained with a universal testing machine. The fractures were classified as reparable or irreparable. Fracture strength values were analyzed with 2-way and 1-way ANOVA (α = 0.05) and Tukey's test. Only the IDS factor was significant (p = 0.001). The teeth restored with IDS (2875 ± 508 N) achieved significantly higher fracture strength than the teeth restored without IDS (2263 ± 354 N) (p < 0.05). The fracture strength of the specimens with IDS did not differ significantly from that of the sound teeth (3230 ± 620 N). The fractures were predominantly reparable. IDS improved the fracture strength of ultrathin occlusal veneers made of PICN.