Purpose: To investigate the marginal and internal adaptations of ceramic inlay restorations placed with immediate dentin sealing (IDS) vs delayed dentin sealing (DDS) procedures. Materials and Methods: Mesial and distal Class II cavities were prepared in 12 extracted molar teeth, which were randomly allocated into six groups of 2 teeth each. Lava Ultimate inlays were fabricated and luted to the cavities using All-Bond universal adhesive system and eCEMENT dual-curing resin cement following IDS/immediate cementation (control groups 1 and 2), IDS/delayed cementation (groups 3 and 4), or DDS/delayed cementation (groups 5 and 6) protocols. Teeth in groups 2, 4, and 6 were subjected to thermocycling of 500 cycles between 5°C and 55°C after inlay cementation. Following staining with silver nitrate solution, the marginal and internal gap volumes were determined using microcomputed tomography images. Statistical analyses were conducted using independent t test and one-way analysis of variance followed by Tukey post hoc test (P .05). Results: Marginal gap volume for DDS (1.856 ± 0.323 mm3) was significantly higher than that of IDS immediately after inlay cementation (0.891 ± 0.281 mm3) (P = .025). Following thermocycling, the internal gap volume for DDS (0.838 ± 0.248 mm3) was significantly higher than that for IDS (0.098 ± 0.066 mm3) (P = .000), but the marginal gap volume of DDS (1.964 ± 0.956 mm3) was not significantly different from that of IDS (1.426 ± 0.725 mm3) (P = .622). Conclusion: Luted ceramic inlays have a superior marginal adaptation right after cementation and a superior internal adaptation after thermocycling when using the IDS technique compared to the DDS technique. However, marginal adaptation after thermocycling was not significantly different between the two techniques.