The conversion of oleic acid (C18:1) to linoleic acid (C18:2) in the endoplasmic reticulum is critical to the accumulation of polyunsaturated fatty acids in seeds and other tissues, and this reaction is catalyzed by a 12-desaturase, FATTY ACID DESATURASE2 (FAD2). Here, we report that the tomato (Solanum lycopersicum) genome harbors two genes, SlFAD2-1 and SlFAD2-2, which encode proteins with in vitro 12-desaturase activity. In addition, tomato has seven divergent FAD2 members that lack 12-desaturase activity and differ from canonical FAD2 enzymes at multiple amino acid positions important to enzyme function. Whereas SlFAD2-1 and SlFAD2-2 are downregulated by biotic stress, the majority of divergent FAD2 genes in tomato are upregulated by one or more stresses. In particular, SlFAD2-7 is induced by the potato aphid (Macrosiphum euphorbiae) and has elevated constitutive expression levels in suppressor of prosystemin-mediated responses2 (spr2), a tomato mutant with enhanced aphid resistance and altered fatty acid profiles. Virus-induced gene silencing of SlFAD2-7 in spr2 results in significant increases in aphid population growth, indicating that a divergent FAD2 gene contributes to aphid resistance in this genotype. Thus, the FAD2 gene family in tomato is important both to primary fatty acid metabolism and to responses to biotic stress.