Polyunsaturated fatty acids (PUFAs) have been identified as key nutrients for the common octopus (Octopus vulgaris), particularly for its early life-cycle stages (paralarvae). Our overarching aim is to identify the dietary essential fatty acid (FA) for octopus paralarvae through characterisation of the enzymes of endogenous PUFA biosynthetic pathways. Here we report on the molecular cloning and functional characterisation of a cDNA encoding a putative elongase of very long-chain fatty acids (Elovl), a critical enzyme that catalyses the elongation of FA including PUFA. Our results suggest that the octopus Elovl is phylogenetically related to Elovl5 and Elovl2, two elongases with demonstrated roles in PUFA biosynthesis in vertebrates. Further evidence supporting a role of the octopus Elovl in PUFA biosynthesis was provided through functional characterisation of its activity in yeast. It was confirmed that expression of the octopus Elovl conferred on yeast the ability to elongate some C18 and C20 PUFAs, while C22 PUFA substrates remained unmodified. Therefore, the substrate specificities exhibited by the octopus elongase were consistent with those of vertebrate Elovl5. Interestingly, the octopus Elovl elongated n − 6 PUFA substrates more efficiently than their homologous n − 3 substrates, suggesting that n − 6 PUFA may have particular biological significance in O. vulgaris. Finally, we investigated the potential role of the newly cloned Elovl in the biosynthesis of non-methylene-interrupted FA, compounds typically found in marine invertebrates and confirmed to be also present in the common octopus.