Purpose: This study aims to develop and characterize copper-doped bioactive glass nanoparticles (BG/CuNp), and to evaluate the effects of their addition into a resin composite on antimicrobial activity (AMA), cytotoxicity (CTX), ultimate tensile strength (UTS), Knoop microhardness (KHN), as well as immediate resin-dentin microtensile bond strength (μTBS), nanoleakage (NL) and in-situ degree of conversion (DC). Materials and Methods: BG/CuNp were added to a resin composite at different concentrations (0% [control]; 5, 10 and 20 wt%). The AMA was evaluated against Streptococcus mutans. For CTX, the Gingival mesenchymal stem cells (GMSC) cell line was used. For UTS and KHN, specimens were tested after 24 h and 28 days. For bonding evaluation, a universal adhesive was applied on flat dentin surfaces, experimental resin composite build-ups were prepared, and specimens were sectioned to obtain resin–dentin sticks. These were evaluated for μTBS, NL and DC after water storage. Data were submitted to statistical analyses (α = 0.05). Results: The addition of 5% and 10% of BG/CuNp increases AMA (P < 0.05), while the CTX remained unchanged with resin-containing BG/CuNp (P > 0.05). UTS and KHN remained stable with the addition of 5% and 10% of BG/CuNp at 24 h, but showed significantly higher values compared to the control after 28 d (P < 0.05). μTBS and in-situ DC remained unchanged with BG/CuNp addition, regardless of the concentration added. However, significantly lower NL was observed for BG/CuNp groups (P < 0.05). Conclusion: The addition of BG/CuNp in the tested concentrations into a resin composite may be an alternative to provide antimicrobial activity and improve the integrity of the hybrid layer, without compromising biological, adhesives and mechanical properties.