Background <p>This study investigated the effect of a 45% carbamide peroxide-containing office-type bleaching agent on the microhardness (MH) and surface roughness (SR) of flowable and paste-type bulk-fill composite resins.</p> Methods <p>This study used two bulk-fill composite resins, 3&#xa0;M Filtek Bulk-Fill (paste) and Estelite Bulk-Fill Flow (flowable). Twenty disc-shaped specimens were prepared and divided into two groups according to the type of bulk-fill materials. All samples were stored at room temperature for 24&#xa0;h with artificial saliva. Initial MH and surface SR values were measured. High-concentration office-type (HCOT) bleaching agent was applied to the samples for 30&#xa0;min, and then the same procedure was repeated after one week. Following completion of bleaching procedures, final MH and surface SR values were evaluated. The MH values were analyzed by ANOVA and Kruskal-Wallis tests was used for SR values with a significance of 0.05.</p> Results <p>A statistically significant difference was found between the MH values of the groups before and after bleaching (<i>p</i> = .002). SR value of 3&#xa0;M Filtek Bulk-Fill was found statistically higher than Estelite Bulk-Fill Flow after or before bleaching treatment. Statistically significant difference was found SR value of 3&#xa0;M Filtek Bulk-Fill when compared to before and after bleaching procedures (<i>p</i> = .024).</p> Conclusions <p>Bleaching with high-concentration office-type material significantly reduced the MH of both bulk-fill composites. While HCOT was applied, Estelite Bulk-Fill Flow did not cause a significant change in surface SR. In contrast, the SR value of 3&#xa0;M Filtek Bulk-Fill was statistically higher. Therefore, if bleaching with a high-concentration office-type bleaching material is planned, it is recommended not to use 3&#xa0;M Filtek Bulk-Fill as a restorative material, or it may be recommended to renew existing composite resin restorations after bleaching treatment.</p>