A field study was conducted to assess the resistance of four cabbage varieties to the Diamondback Moth (DBM, Plutella xylostella) under field conditions. The trial took place during the growing season of 2020–2021 at the Agricultural Research Station, University of Basrah, Iraq. The results revealed statistically significant differences in the DBM population density between the cabbage varieties. The red cabbage varieties Hanar and Kirmizi exhibited significantly lower densities of larvae and pupae compared with the white variety Dalal, which showed the highest infestation levels. This study explored the role of morphological traits and phytochemicals—including plant pigments (chlorophyll a, b, total, carotene, and anthocyanin), proteins, carbohydrates, phenols, and glucosinolates—in cabbage's defense against DBM. The red varieties Hanar and Kirmizi were found to have a thick cuticle, epidermal layers, and parenchymal cells compared with the susceptible variety Dalal. The glucosinolate profiles of the studied cabbage varieties revealed several compounds, including progoitrin, glucoraphanin, sinigrin, gluconapin, glucoiberverin, glucoberferoin, glucoalyssin, goitrin, and sulforaphaneolate. The lower infestation levels on Kirmizi correlated with its higher concentrations of anthocyanin, carotene, phenols, and the glucosinolates progoitrin and goitrin. This study highlights the importance of selecting cabbage varieties with specific anatomical traits as a key element in integrated pest management (IPM) strategies to prevent DBM infestation.