Bacteriophages are viruses that have the potential to combat bacterial infections caused by antimicrobial-resistant bacterial strains. In this study, we investigated a novel lytic bacteriophage, vB_EcoS_JSSK01, isolated from sewage in Hualien, Taiwan, which effectively combats multidrug-resistant (MDR) of the K1 capsular type. K1 is a major cause of severe extraintestinal infections, such as neonatal meningitis and urinary tract infections. Phage JSSK01 was found to have a genome size of 44,509 base pairs, producing approximately 123 particles per infected cell in 35 min, and was highly stable across a range of temperatures and pH. JSSK01 infected 59.3% of the MDR strains tested, and its depolymerase (ORF40) specifically degraded the K1 capsule in these bacteria. In a zebrafish model, JSSK01 treatment after infection significantly improved survival, with survival in the treated group reaching 100%, while that in the untreated group dropped to 10% after three days. The functional activity of depolymerase was validated using zone inhibition and agglutination tests. These results indicate that JSSK01 and its substrate-specific depolymerase have promising therapeutic and diagnostic applications against K1-encapsulated MDR infections.