Background <p>Seed priming is an essential technical tool in agricultural production to solve the problem of slow seedling emergence and irregular seedling emergence. Salt stress significantly inhibits plant growth and development. Although Siberian wildrye (<i>Elymus sibiricus</i> L.) exhibits strong salt tolerance, the molecular mechanisms underlying its adaptation to high-salt stress require further investigation. This study aimed to elucidate how spermidine seed priming improves germination and seedling growth of <i>Elymus sibiricus</i> under salt stress by integrating physiological traits with transcriptomic and metabolomic profiles.</p> Methods <p>In this study, the <i>E. sibiricus</i> cv. ‘Lanyu No.2’ was utilized as experimental material. Seeds were soaked in four different concentrations of spermidine solution and germinated for 11 days under 200 mM NaCl to investigate the effects of spermidine priming on the germination stage. Additionally, seedlings grown normally to 60 days were subjected to 14 days of salt stress treatment to examine the effects of spermidine priming on the seedling stage.</p> Results <p>The results of this study indicate that under salt stress, 0.50 mM spermidine priming increased seed germination rates and bioaccumulation levels. To reveal the mechanism of spermidine priming alleviating the salt stress, the chlorophyll and carotenoid content, antioxidant enzyme activity, and osmotic regulatory substance content were detected. Our results demonstrated that seedlings from 0.50 mM spermidine-primed seeds showed better photosynthetic efficiency and a stronger ability to scavenge reactive oxygen species. Besides, transcription sequencing analysis, it was found that almost all the photosynthesis-related genes in the spermidine-primed seedlings were up-regulated after 14 days of salt stress treatment. It might be a way to enhance the photosynthetic rate to adapt to salt stress in <i>E. sibiricus</i>. Weighted gene co-expression network analysis (WGCNA) identified three hub genes that may be involved in <i>E. sibiricus</i> salt response. Additionally, integrated transcriptomes and metabolomes analyses revealed that spermidine may enhance salt tolerance in <i>E. sibiricus</i> seedlings through the phenylpropanoid biosynthesis and glyoxylate and dicarboxylate metabolism pathways.</p> Conclusions <p>Our findings provide candidate pathways and markers that could be exploited in breeding and seed technology for salt-tolerant forage grasses.</p>