Objective: This study aimed to develop an antibiotic-free, multifunctional antibacterial hydrogel dressing that integrates rapid gelation, robust tissue adhesion, controllable self-degradation, fluorescence-enabled visualization, and broad-spectrum antibacterial activity, and to systematically evaluate its in vitro performance. Methods: An ultrafast in situ-forming isoindole-crosslinked hydrogel dressing, designated OSN, was fabricated through a ternary condensation reaction initiated by mixing precursor solutions of o-phthalaldehyde(OPA), four-arm poly(ethylene glycol) thiol(tetra-PEG-SH), and four-arm poly(ethylene glycol) amine (tetra-PEG-NH₂). By varying the molar ratio of OPA to the two polymer precursors (0.5:1:1, 1:1:1, and 2:1:1), three hydrogels, denoted OSN-1, OSN-2, and OSN-3, were prepared. Their gelation behavior, microstructure, mechanical properties, self-degradation profile, biocompatibility, and antibacterial activity were systematically characterized using scanning electron microscopy, rheological analysis, mechanical testing, UV-visible/fluorescence spectroscopy, cell-based assays (including CCK-8, live/dead staining, and scratch wound assays) and antibacterial evaluations (including colony counting, scanning electron microscopy, biofilm staining and flow cytometry). Results: The OSN hydrogels formed rapidly in situ within 10 s and exhibited a homogeneous three-dimensional network architecture. Increasing the OPA content accelerated gelation and imparted self-degradable behavior to the hydrogel matrix, with OSN-3 undergoing complete degradation within 24 h. The hydrogels demonstrated favorable tissue-adhesive strength, together with good biocompatibility. They also promoted the migration of L929 fibroblasts and exhibited potent antibacterial activity against both methicillin-resistant Staphylococcus aureus and Escherichia coli, effectively disrupting bacterial biofilms. In addition, degradation of the hydrogel was accompanied by attenuation and eventual disappearance of blue fluorescence, attributable to cleavage of the isoindole ring, indicating the potential for fluorescence-based visualization of the degradation process. Conclusion: This study successfully developed an antibiotic-free antibacterial OSN hydrogel dressing based on OPA-mediated ternary condensation. The resulting dressing combines injectability, rapid gelation, strong tissue adhesion, controllable self-degradation, fluorescence-enabled visualization, and broad-spectrum antibacterial activity, while maintaining good biocompatibility. These findings provide a novel and promising strategy for the development of antibiotic-free intelligent wound-management strategies.