Objectives This study aims to compare the performance of a bulk-fill and a nanofill resin composite in class II restorations after 6 years. Materials and methods Fifty patients having at least two class II carious lesions were recruited for the study. One lesion in each patient was randomly assigned to be restored using either the Tetric EvoCeram Bulk Fill (TB) or Filtek Ultimate (FU) resin composites with their respective adhesives. One hundred four restorations were placed by two calibrated operators. Restorations were evaluated at baseline and annually over the course of 6 years by two examiners using modified USPHS criteria. Data were statistically analyzed using the Chi-square and Cochran Q tests (p < 0.05). Results Sixty-six restorations in 33 patients were evaluated after 6 years. Only one restoration was lost from FU group at 5 years. At the end of 6 years, marginal discoloration was observed in three (9.1%) TB and eight (36.4%) FU restorations creating a significant difference between the groups (p < 0.05). The FU group showed a significant increase in marginal discoloration at 6 years from the baseline (p < 0.05). Marginal adaptation was rated as Bravo for 9.1% and 24.2% of TB and FU restorations, respectively (p > 0.05). Significant degradation was observed within each group in terms of marginal adaptation (p < 0.05). There were no statistically significant differences between the groups for the other criteria tested (p > 0.05). Conclusions Bulk-fill restorations performed better for marginal discoloration. The remaining clinical performance criteria of bulk-fill and nanofill resin composite restorations were similar after 6 years.