Abstract Temperature is one of the defining parameters of an ecological niche, and ambient temperature change is a physiological challenge faced by all living cells. Most organisms are adapted to growing within a temperature range that rarely exceeds ~ 30°C, but the anaerobic thermophilic bacterium Kosmotoga olearia is capable of growing over an extremely wide temperature range (20°C - 79°C). To pinpoint genomic determinants of this flexible phenotype, we compared transcriptomes of K. olearia cultures grown at its optimal 65°C to those at 30°C, 40°C, and 77°C. We found that changes in temperature significantly affect expression of 573 of 2,224 K. olearia genes. At different temperatures K. olearia remodels its metabolism dramatically, with increased expression of genes involved in energy and carbohydrate metabolism at high temperatures and up-regulation of amino acid metabolism at lower temperatures. At sub-optimal temperatures, many transcriptional changes were similar to those observed in mesophilic bacteria at physiologically low temperatures, including up-regulation of genes encoding enzymes for fatty acid synthesis, typical cold stress genes, and ribosomal proteins. In comparison to other Thermotogae, K. olearia has multiple copies of some cold-associated genes, suggesting that an increase in gene copy number is a strategy for cold adaptation. Many of these cold response genes are predicted to be laterally acquired, highlighting the role of gene exchange in bacterial thermoadaptation. Notably, at 77°C one third of the up-regulated genes encode proteins with hypothetical functions, indicating that many features of adaptations to high temperature growth are still unknown. Importance The subsurface is arguably the largest habitat on Earth, and insights into the composition, adaptation, and evolution of its microbial communities is likely to advance our knowledge of the biosphere and of global element cycling. Some subsurface systems, such as oil reservoirs, are thought to be so stable that any changes in environmental conditions, including temperature, occur over thousands of years. Yet a member of the oil reservoir microbial communities, the bacterium Kosmotoga olearia, is capable of growing over a range of 59°C. Our finding of coordinated temperature-specific gene expression patterns, and by extension temperature specific metabolism, suggests that Kosmotoga populations encounter variable environments, probably through migration. This raises the question: are deep subsurface microbial communities more dynamic than currently perceived? The large number of identified temperature-responsive genes also indicates that temperature response is a complex polygenic trait.