Abstract Background Fatty acids are the building blocks of complex lipids essential for living organisms. In mosquitoes, fatty acids are involved in cell membrane production, energy conservation and expenditure, innate immunity, development, and reproduction. Fatty acids are synthesized by a multifunctional enzyme complex called fatty acid synthase (FAS). Several paralogues of FAS were found in the Aedes aegypti ( Ae. aegypti ) mosquito. However, the molecular characteristics and the expression of some of these paralogues have not been investigated. Methods Genome assemblies of Ae. aegypti were analyzed and orthologues of human FAS were identified. Phylogenetic analysis and in silico molecular characterization were performed to identify the functional domains of the Ae. aegypti FAS ( Aa FAS). Quantitative analysis and loss-of-function experiments were performed to determine the significance of different Aa FAS transcripts in various stages of development, expression following different diets and the impact of Aa FAS on dengue virus, serotype 2 (DENV2) infection and transmission. Results We identified seven putative FAS genes in the Ae. aegypti genome assembly, based on nucleotide similarity to the FAS proteins (tBLASTn) of humans, other mosquitoes and invertebrates. Bioinformatics and molecular analyses suggested that only five of the FAS genes produce mRNA and therefore represent complete gene models. Expression levels of Aa FAS varied among developmental stages and between male and female Ae. aegypti . Quantitative analyses revealed that expression of Aa FAS1, the putative orthologue of the human FAS, was highest in adult females. Transient knockdown (KD) of Aa FAS1 did not induce a complete compensation by other Aa FAS genes but limited DENV2 infection of Aag2 cells in culture and the midgut of the mosquito. Conclusion Aa FAS1 is the predominant Aa FAS in the adult mosquitoes. It has the highest amino acid similarity to human FAS and contains all enzymatic domains typical of human FAS. Aa FAS1 also facilitated DENV2 replication in both cell culture and in mosquito midguts. Our data suggest that Aa FAS1 may play a role in transmission of dengue viruses, and could represent a target for intervention strategies.