The cuticle plays an important role in plant interactions with pathogens and with their surroundings. The cuticle acts as both a physical barrier against physical stresses and pathogens and a chemical deterrent and activator of the plant defense response. Cuticle production in tomato plants is regulated by several transcription factors, including SlSHINE3, an ortholog of the Arabidopsis WIN/SHN3. Here we used a SlSHINE3-overexpressing (SlSHN3-OE) and silenced (Slshn3-RNAi) lines and a mutant in SlCYP86A69 (Slcyp86A69)--a direct target of SlSHN3--to analyze the roles of the leaf cuticle and cutin content and composition in the tomato plant's defense response to the necrotrophic foliar pathogen Botrytis cinerea and the biotrophic bacterial pathogen Xanthomonas campestris pv. vesicatoria. We showed that SlSHN3, which is predominantly expressed in tomato fruit epidermis, also affects tomato leaf cuticle, as morphological alterations in the SlSHN3-OE leaf tissue resulted in shiny, stunted and permeable leaves. SlSHN3-OE leaves accumulated 38% more cutin monomers than wild-type leaves, while Slshn3-RNAi and Slcyp86A69 plants showed a 40 and 70% decrease in leaf cutin monomers, respectively. Overexpression of SlSHN3 resulted in resistance to B. cinerea infection and to X. campestris pv. vesicatoria, correlated with cuticle permeability and elevated expression of pathogenesis-related genes PR1a and AOS. Further analysis revealed that B. cinerea-infected Slshn3-RNAi plants are more sensitive to B. cinerea and produce more hydrogen peroxide than wild-type plants. Cutin monomer content and composition differed between SlSHN3-OE, Slcyp86A69, Slshn3-RNAi and wild-type plants, and cutin monomer extracted from SlSHN3-OE plants altered the expression of pathogenesis-related genes in wild-type plants.