Crop wild relatives (CWRs) are close weedy relatives of plant species and are generally underestimated because of their undesirable characters and smaller yield potential. However, these species carry many useful alleles and agronomic traits; therefore, they could serve as significant resources for the transfer of such genes to cultivated germplasms. Sinapis alba L. or white mustard is a member of the Brassicaceae family and a CWR of the oilseed Brassica spp., which possesses resistance to biotic/abiotic stresses like Alternaria blight and drought. Attempts to introgress such traits from S. alba to cultivated rapeseed and mustard germplasms have largely been unsuccessful, which necessitates the utilization of modern biotechnological tools to develop resistant cultivars. In our study, we validated the resistance of S. alba against the devastating necrotrophic fungal pathogen Alternaria brassicicola, and we characterized the interaction between the two. Subsequently, we performed transcriptomic analysis to dissect the defense responses of S. alba against the pathogen. The study established S. alba to be resistant to the Alternaria pathogen and presented new insights into its defense machinery in the form of a comprehensive dataset of genes and transcription factors modulated specifically in response to A. brassicicola, paving a way towards the development of Alternaria-resistant oilseed Brassicas.