With the aim of identifying novel thermostable esterases, comprehensive sequence databases and cloned fosmid libraries of metagenomes derived from an offshore oil reservoir on the Norwegian Continental Shelf were screened for enzyme candidates using both sequence-and function-based screening. From several candidates identified in both approaches, one enzyme discovered by the functional approach was verified as a novel esterase and subjected to a deeper characterization. The enzyme was successfully over-produced in Escherichia coli and was shown to be thermostable up to 90°C, with the highest esterase activity on short-chain ester substrates and with tolerance to solvents and metal ions. The fact that the thermostable enzyme was solely found by functional screening of the oil reservoir metagenomes illustrates the importance of this approach as a complement to purely sequence-based screening, in which the enzyme candidate was not detected. In addition, this example indicates the large potential of deep-sub-surface oil reservoir metagenomes as a source of novel, thermostable enzymes of potential relevance for industrial applications.