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Rapeseed (<i>Brassica napus</i> L.) is an important oil crop and a major source of tocopherols, also known as vitamin E, in human nutrition. Enhancing the quality and composition of fatty acids (FAs) and tocopherols in seeds has long been a target for rapeseed breeding. The gene <i>γ-Tocopherol methyltransferase</i> (<i>γ-TMT</i>) encodes an enzyme catalysing the conversion of γ-tocopherol to α-tocopherol, which has the highest biological activity. However, the genetic basis of <i>γ-TMT</i> in <i>B. napus</i> seeds remains unclear. In the present study, <i>BnaC02.TMT.a</i>, one paralogue of <i>Brassica napus γ-TMT</i>, was isolated from the <i>B. napus</i> cultivar “Zhongshuang11” by nested PCR, and two homozygous transgenic overexpression lines were further characterised. Our results demonstrated that the overexpression of <i>BnaC02.TMT.a</i> mediated an increase in the α- and total tocopherol content in transgenic <i>B. napus</i> seeds. Interestingly, the FA composition was also altered in the transgenic plants; a reduction in the levels of oleic acid and an increase in the levels of linoleic acid and linolenic acid were observed. Consistently, <i>BnaC02.TMT.a</i> promoted the expression of <i>BnFAD2</i> and <i>BnFAD3</i>, which are involved in the biosynthesis of polyunsaturated fatty acids during seed development. In addition, <i>BnaC02.TMT.a</i> enhanced the tolerance to salt stress by scavenging reactive oxygen species (ROS) during seed germination in <i>B. napus</i>. Our results suggest that <i>BnaC02.TMT.a</i> could affect the tocopherol content and FA composition and play a positive role in regulating the rapeseed response to salt stress by modulating the ROS scavenging system. This study broadens our understanding of the function of the <i>Bnγ-TMT</i> gene and provides a novel strategy for genetic engineering in rapeseed breeding.