The ever-growing challenges of traditional antibiotic therapy and chronic wound healing have created a hot topic for the development and application of new antimicrobial agents. Silver nanoclusters (Ag NCs) with ultrasmall sizes (<2 nm) and antibacterial effects are promising candidates for next-generation antibiotics, particularly against multi-drug resistant strains. However, the biosafety in the clinical application of Ag NCs remains suboptimal despite some existing studies of Ag NCs for biomedical applications. Considering this, an ultrasmall Ag NC with excellent water solubility was synthesized by a two-phase ligand-exchange method, which exhibits broad-spectrum antibacterial performance. The minimum inhibitory concentrations of Ag NCs against MRSA, S. aureus, P. aeruginosa and E. coli were evaluated as 50, 80, 5 and 5 μg mL-1, respectively. Furthermore, a carbomer hydrogel was prepared to be incorporated into the Ag NCs for achieving excellent biocompatibility and biosafety. In vitro experiments demonstrate that the Ag NC-gel exhibits good antibacterial properties with lower cytotoxicity. Finally, in vivo experiments suggest that this ultrasmall Ag NC functionalized with the hydrogel can serve as an effective and safe antimicrobial agent to aid in wound healing.