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Very‐long‐chain fatty acids (VLCFAs) are essential precursors for plant membrane lipids, cuticular waxes, suberin, and storage oils. Integral to the fatty acid elongase (FAE) complex, 3‐ketoacyl‐CoA synthases (KCSs) function as crucial enzymes in the VLCFA pathway, determining the chain length of VLCFA. This study explores the in‐planta role of the KCS19 gene. KCS19 is predominantly expressed in leaves and stem epidermis, sepals, styles, early silique walls, beaks, pedicels, and mature embryos. Localized in the endoplasmic reticulum, KCS19 interacts with other FAE proteins. kcs19 knockout mutants displayed reduced total wax and wax crystals, particularly alkanes, while KCS19 overexpression increased these components and wax crystals. Moreover, the cuticle permeability was higher for the kcs19 mutants compared to the wild type, rendering them more susceptible to drought and salt stress, whereas KCS19 overexpression enhanced drought and salt tolerance. Disrupting KCS19 increased C18 species and decreased C20 and longer species in seed fatty acids, indicating its role in elongating C18 to C20 VLCFAs, potentially up to C24 for seed storage lipids. Collectively, KCS19‐mediated VLCFA synthesis is required for cuticular wax biosynthesis and seed storage lipids, impacting plant responses to abiotic stress. Summary Statement: KCS19, encoding a fatty acid elongase 3‐ketoacyl‐CoA synthase, plays a crucial role in synthesizing very‐long‐chain fatty acids for cuticular wax and seed storage lipids in Arabidopsis. Its function significantly impacts plant responses to drought and salt stress. [ABSTRACT FROM AUTHOR]