Salmonella4,[5],12:i:-, a monophasic variant ofS. Typhimurium, has become a global serovar causing animal and human infections since its first emergence in the late 1980's. Several previous studies showed the increasing prevalence ofS. 4,[5],12:i:- in China, most of which were from swine with multidrug resistance (MDR) profiles. However, the molecular characteristic and evolution ofS. 4,[5],12:i:- in the same swine farm are still unknown. In this study, a total of 54S. entericastrains were isolated from different fattening pigs aged 1, 3, and 6 months, most of which belonged toS. 4,[5],12:i:-. Whole-genome sequencing revealed that all 45S. 4,[5],12:i:- strains belonged to ST34 and were further divided into two different ribosomal STs and nine different core-genome STs. Phylogenetic analysis of 286S. 4,[5],12:i:- strains in China, including 241 from the EnteroBaseSalmonelladatabase, revealed the genetic diversity ofS. 4,[5],12:i:- and indicated thatS. 4,[5],12:i:- in this swine farm might have multiple origins. Three different IncHI2 plasmids carrying various resistance genes were characterized by nanopore sequencing and could be conjugated toEscherichia coli. The colistin resistance genemcr-1and ESBLs geneblaCTX − M−14were co-located on the chromosome of one strain. The dynamic changes in antimicrobial resistance regions and transferability of IncHI2 plasmids, as well as the chromosomal location of resistance genes, facilitated the diversity of the antimicrobial resistance characteristics inS. 4,[5],12:i:-. Since the swine farm is regarded as the important reservoir of MDRS. 4,[5],12:i:-, the prevalence and evolution ofS. 4,[5],12:i:- from swine farms to pig products and humans should be continually monitored.