ABSTRACTAcinetobacter baumanniicauses difficult-to-treat infections mostly among immunocompromised patients. Clinically relevantA. baumanniilineages and their carbapenem resistance mechanisms are sparsely described in Nigeria. This study aimed to characterize the diversity and genetic mechanisms of carbapenem resistance amongA. baumanniistrains isolated from hospitals in southwestern Nigeria. We sequenced the genomes of allA. baumanniiisolates submitted to Nigeria’s antimicrobial resistance surveillance reference laboratory between 2016 and 2020 on an Illumina platform and performedin silicogenomic characterization. Selected strains were sequenced using the Oxford Nanopore technology to characterize the genetic context of carbapenem resistance genes. The 86A. baumanniiisolates were phylogenetically diverse and belonged to 35 distinct Oxford sequence types (oxfSTs), 16 of which were novel, and 28 Institut Pasteur STs (pasSTs). Thirty-eight (44.2%) isolates belonged to none of the known international clones (ICs). Over 50% of the isolates were phenotypically resistant to 10 of 12 tested antimicrobials. The majority (n= 54) of the isolates were carbapenem resistant, particularly the IC7 (pasST25; 100%) and IC9 (pasST85; >91.7%) strains.blaOXA-23(34.9%) andblaNDM-1(27.9%) were the most common carbapenem resistance genes detected. AllblaOXA-23genes were carried on Tn2006or Tn2006-like transposons. Our findings suggest that a 10-kb Tn125composite transposon is the primary means ofblaNDM-1dissemination. Our findings highlight an increase inblaNDM-1prevalence and the widespread transposon-facilitated dissemination of carbapenemase genes in diverseA. baumanniilineages in southwestern Nigeria. We make the case for improving surveillance of these pathogens in Nigeria and other understudied settings.IMPORTANCEAcinetobacter baumanniibacteria are increasingly clinically relevant due to their propensity to harbor genes conferring resistance to multiple antimicrobials, as well as their ability to persist and disseminate in hospital environments and cause difficult-to-treat nosocomial infections. Little is known about the molecular epidemiology and antimicrobial resistance profiles of these organisms in Nigeria, largely due to limited capacity for their isolation, identification, and antimicrobial susceptibility testing. Our study characterized the diversity and antimicrobial resistance profiles of clinicalA. baumanniiin southwestern Nigeria using whole-genome sequencing. We also identified the key genetic elements facilitating the dissemination of carbapenem resistance genes within this species. This study provides key insights into the clinical burden and population dynamics ofA. baumanniiin hospitals in Nigeria and highlights the importance of routine whole-genome sequencing-based surveillance of this and other previously understudied pathogens in Nigeria and other similar settings.