We present an antibacterial polymersome-hydrogel composite that combines the advantages of both antimicrobial peptides and antibiotics. The polymersomes are self-assembled from poly(ε-caprolactone)-block-poly(lysine-stat-phenylalanine) [PCL-b-P(Lys-stat-Phe)], exhibiting long-acting intrinsic antibacterial capabilities against both Gram-positive and Gram-negative bacteria. In addition, penicillin is encapsulated into the polymersomes, showing quick and enhanced antibacterial activities. Furthermore, a dynamic hydrogel network is prepared based on the Schiff base linkages between the aldehyde groups of dibenzaldehyde-functionalized PEG (DF-PEG) and the amino groups of chitosan. During this process, the penicillin-loaded polymersomes are grafted into the hydrogel networks by Schiff base linkages to afford polymersome-hydrogel composites, which exhibit two-stage antibacterial behavior: (1) penicillin can be released from the hydrogel networks for quick antibacterial function; (2) the polymersomes and the remaining penicillin can be further released for long-term antibacterial effects. Overall, this polymersome-hydrogel composite represents a new type of injectable antibacterial biomaterial for quick and long-acting antibacterial applications.