Purpose: The aim of this in vitro study was to evaluate the marginal adaptation and retention of inlay fixed partial dentures (IFPDs) made with 1 fiber-reinforced composite and 2 different ceramic materials using quantitative scanning electron microscope analysis after thermal cycling and mechanical loading, which simulated approximately 5 years of oral service. Materials and Methods: Eighteen IFPDs made with fiber-reinforced composite (SR Adoro/Vectris), zirconium oxide-TZP (Cercon), and magnesia partially stabilized zirconia (DC-Leolux) covered with silica-based ceramics were tested in this study. The specimens were mechanically loaded in the vestibular cusp of the pontic element in a computer-controlled masticator with 1,200,000 half-sinusoid mechanical cycles of maximum 49 N each at a frequency of 1.7 Hz. A total of 3,000 thermocycles at 5°C and 55°C, 2 minutes each, were performed simultaneously. The marginal adaptation was analyzed at the interface of the luting composite and the abutment inlay/onlay (CI) and at the interface of the tooth and the luting composite (TC). Results: The percentages of continuous margin at the CI interface were 94.6 ± 3.1 and 88 ± 6.7 for Adoro/Vectris, 92.9 ± 5 and 85.7 ± 6.1 for Cercon, and 96.2 ± 2.1 and 82.2 ± 9.8 for DC-Leolux, respectively, before and after loading. The percentages of continuous margin at the TC interface were 86.7 ± 6.7 and 62.5 ± 16.4 for Adoro/Vectris, 93.3 ± 3.4 and 83.2 ± 5.9 for Cercon, and 96.1 ± 2.4 and 75.3 ± 7 for DC-Leolux. Statistically significant differences were found after loading between the fiber-reinforced composite and the 2 ceramic systems at the TC interface. Conclusion: Within the limitations of this experimental study with regard to the sample size and contacting vectors, the results showed that flexibility of the framework may play an important role in the marginal adaptation of the IFPDs. More rigid materials may transfer less stress to the margins, thus promoting a more stable adhesion to the dental tissues.