Salt stress severely affects the growth of turnips (Brassica rapa subsp. rapa), leading to a decline in quality and a reduction in yield. Methyl jasmonate is an endogenous plant hormone that plays a role in regulating plant responses to salt stress. However, its role and mechanism in regulating the response of turnip salt stress remain unclear. Herein, exogenous 100 μM MeJA was applied to four-leaf turnip seedlings subjected to 100 mM NaCl stress to investigate the changes in growth parameters, plant physiology, gene expression, and hormone accumulation after treatment for 3, 5, and 7 days. The results indicated that exogenous spraying of MeJA restricted the growth of turnip seedlings, but enhanced the activity of antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), and ascorbate peroxidase (APX), while reducing the generation of hydrogen peroxide (H₂O₂). Additionally, RNA-seq data showed that exogenous MeJA significantly up-regulates the expression of genes associated with resistance to abiotic stress, specifically those involved in sulfur metabolism, phytohormone signaling, glutathione metabolism, and phenylpropanoid biosynthesis. This up-regulation improved the activity of antioxidant enzymes in vivo, facilitated the scavenging of accumulated reactive oxygen species, and strengthened the plant’s defense mechanisms. In summary, exogenous application of MeJA inhibited the growth of turnip seedlings, but enhanced the plant’s physiological responses to salt stress.