Purpose: To evaluate the effect of water aging of adherend composite on repair bond strength to nanofilled composites with specific fillers using different bonding agents. Materials and Methods: Three nanofilled composites - Beautifil II with S-PRG filler (BE) / Filtek Supreme ultra with nanocluster filler (SP) / Estelite Σ Quick (ES) - and one microhybrid composite, Clearfil APX (AP), were used in this study. The composite disks were immersed in water for different durations (immediate, 1 week, 2 weeks or 1 month), and then the polished surfaces were treated with one of three bonding agents - no treatment (control), application of Clearfil SE One (SE), application of Clearfil SE One plus Clearfil Porcelain Bond Activator (PB) - then filled with a repair composite. The bonded composite disks were subjected to the microshear bond strength (µSBS) test. Additionally, water sorption (W_sp) and solubility (W_sl) of the resin composite were measured. The µSBS data were was statistically analyzed using a three-way ANOVA and t-test with Bonferroni correction for multiple comparisons. Results: Water aging of adherend composite affected the repair bond strength (p 0.05). For BE, SP, and ES, application of an adhesive agent improved repair bond strengths to water-aged composites (p 0.05), but adding a silane coupling agent could not (p > 0.05). For AP, the µSBS significantly increased, with control group SE group PB group (p 0.05). Conclusion: Microhybrid composite was a more suitable material for composite repair than nanofilled composite, due to adhesion to exposed, larger silica fillers. S-PRG filler and nanocluster filler in the nanofilled composites played a slight role in improving their repair bonding performances with the bonding agents tested. Keywords: bond strength, dental materials, resin composite repair, surface treatments, water solubility, water sorption