Abstract: Citrobacter freundii has been a highly pathogenic pathogen in the breeding of Rana catesbeiana in recent years. Its multi-drug resistance makes the prevention and control of disease difficult. Thus, by isolating a lytic phage NM4.3 specific to C. freundii from bullfrog culture tailwater, we conducted the comprehensive analyses of its whole genome, biological characteristics, and phylogenetic relationships in order to evaluate its potential therapeutic applications. The results indicate that NM4.3 was a circular dsDNA phage with a genome size of 174 351 bp and a GC content of 38.74%. It encoded 310 open reading frames (ORFs), including genes related to virion structure, host lysis, and DNA metabolism, as well as 13 tRNAs. The biological characteristics demonstrate that NM4.3 had a short incubation period (10 min) and a high lytic efficiency (Burst size of 76 PFU·cell⁻¹). NM4.3 retained some activity after being treated at 60 °C and at pH levels ranging from 3 to 12 for 1 hour. Based on the nucleic acid and amino acid sequence analysis of the conserved terminase large subunit (Ter-L) of the genome, NM4.3 was classified as a member of the Straboviridae, Tevenvirinae, Moonvirus, with an average nucleotide identity (ANI) of less than 95% compared to known members, thus defining it as a new species within this family. Genome annotation reveals variations in its long tail fibrin, which may serve as a host-specific factor.