<p>This study aimed to investigate the mechanical behavior of the tooth–class II restoration complex restored with composite resin, ceramic, amalgam, and gold using three-dimensional linear and non-linear finite element analyses (FEA). Previous FEA studies generally assumed linear mechanical properties and bonded interfaces between tooth and restorative materials, which differ from clinical reality, particularly for non-bonded restorations such as amalgam and gold. Therefore, in this study, non-linear mechanical properties and non-bonded contact conditions were adopted for different restorative materials. The results showed that the highest enamel and dentin stresses occurred in amalgam restorations, while the lowest were found in ceramic restorations. Amalgam and gold produced higher stress in enamel and dentin compared with composite resin and ceramic. The higher stress observed in amalgam and gold was mainly associated with the non-bonded (frictional) contact condition, because the stresses in these restorations remained below their reported yield strengths under the applied loading conditions. This study provides mechanical insights into the behavior of the tooth–class II restoration complex and offers a possible mechanical interpretation of previously reported clinical observations. These findings should be interpreted within the limitations of the finite element model and do not directly represent clinical outcomes.</p>