A putative type II toxin-antitoxin (TA) module almost exclusively associated with conjugative IncC plasmids is homologous to the higBA family of TA systems found in chromosomes and plasmids of several species of bacteria. Despite the clinical significance and strong association with high-profile antimicrobial resistance (AMR) genes, the TA systems of IncC plasmids remain largely uncharacterized. In this study, we present evidence that IncC plasmids encode a HigB-like toxin that strongly inhibits bacterial growth and results in cell elongation in E. coli. IncC HigB toxin acts as a ribosome-dependent endoribonuclease that significantly reduces the transcript abundance of a subset of adenine-rich mRNA transcripts such as lpp and ompA. A glycine residue at amino acid position 64 is highly conserved in HigB toxins from different bacterial species and its replacement with valine (G64V) abolishes the toxicity and the mRNA cleavage activity of the HigB toxin carried by IncC plasmids. Exposure to ciprofloxacin, a DNA-damaging fluoroquinolone antibiotic, results in elevated IncC higB expression levels. The IncC higBA TA functions as an effective addiction module that maintains plasmid stability in an antibiotic-free environment. This higBA addiction module is the only TA system we identified in the IncC backbone and appears essential for their stable maintenance and dissemination of AMR genes carried by this plasmid. A better understanding of this higBA-type TA module potentially allows for its subversion as part of an AMR eradication strategy.