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As many bacteria detected in Antarctic environments are neither true psychrophiles nor endemic species, their proliferation in spite of environmental extremes gives rise to genome adaptations. <i>Janthinobacterium</i> sp. CG23_2 is a bacterial isolate from the Cotton Glacier stream, Antarctica. To understand how <i>Janthinobacterium</i> sp. CG23_2 has adapted to its environment, we investigated its genomic traits in comparison to genomes of 35 published <i>Janthinobacterium</i> species. While we hypothesized that genome shrinkage and specialization to narrow ecological niches would be energetically favorable for dwelling in an ephemeral Antarctic stream, the genome of <i>Janthinobacterium</i> sp. CG23_2 was on average 1.7 &#177; 0.6 Mb larger and predicted 1411 &#177; 499 more coding sequences compared to the other <i>Janthinobacterium</i> spp. Putatively identified horizontal gene transfer events contributed 0.92 Mb to the genome size expansion of <i>Janthinobacterium</i> sp. CG23_2. Genes with high copy numbers in the species-specific accessory genome of <i>Janthinobacterium</i> sp. CG23_2 were associated with environmental sensing, locomotion, response and transcriptional regulation, stress response, and mobile elements&#8212;functional categories which also showed molecular adaptation to cold. Our data suggest that genome plasticity and the abundant complementary genes for sensing and responding to the extracellular environment supported the adaptation of <i>Janthinobacterium</i> sp. CG23_2 to this extreme environment.