Recently, the interest in antimicrobial hydrogels with impregnated antibacterial agents has significantly increased because of their ability to combat infection in biomedical applications, including wound management, tissue engineering, and biomaterial surface coating. Among these antibacterial reagents, silver nanoparticles (AgNP) show good antibacterial activity against both gram-negative and gram-positive bacteria, including highly multi-resistant strains. However, the entrapment of AgNP within a hydrogel matrix is often associated with toxicity issues because of the use of chemical reductants (e.g., commonly sodium borohydride), burst leaching, or unwanted agglomeration of AgNP in the absence of surfactants or stabilizers. In this study, we present catechol-rich gelatin hydrogels with in situ hybridization of AgNP for enhanced antimicrobial activities. AgNP were formed through a redox reaction between silver ions and the catechol moieties of a gelatin derivative polymer, without the addition of any chemical reductants. The AgNP with an average size of 20 nm were entrapped within hydrogel matrices and showed sustained release from the hydrogel matrix (8.7% for 14 days). The resulting hydrogels could kill both gram-negative and gram-positive bacteria, depending on the amount of AgNP released from the hydrogels and did not have a significant influence on mammalian cell viability. We believe that our catechol-rich hydrogels in situ hybridizations with AgNP have great potential for biomedical applications, such as wound management and surface coating, because of their excellent antibacterial activities and biocompatibility.