Canola (Brassica napus L.) productivity is severely affected by various biotic and abiotic stresses such as insects, pathogens, drought, and cold. Characterization of genes with specific effects on particular stress responses is one of the long-term goals for B. napus genetic improvement. Here we explored the roles of Brassica napus myrosinase-associated protein 1 (BnMyAP1), which was highly represented (expressed) in a B. napus subtractive library developed after leaf damage by the crucifer flea beetle [Phyllotreta cruciferae (Goeze)]. Expression of BnMyAP1 was under different developmental control in B. napus, but it was co-induced in B. napus by flea beetle feeding, Sclerotinia sclerotiorum (Lib.) de Bary infection, drought, and cold. A total of 20 BnMyAPs were represented in different B. napus stress and development expressed sequence tag libraries and indicated larger, more diversified families than were previously known. Overexpression of BnMyAP1 in Arabidopsis thaliana (L.) Heynh. resulted in an increased survival ratio (63%–90%) compared with that of the controls (15%–20%) after S. sclerotiorum infection, which suggests that BnMyAP1 could be used to protect B. napus oilseed and vegetables from the S. sclerotiorum disease.