Environmental metal exposure is a key risk factor for gestational diabetes mellitus (GDM), but the mechanisms remain unclear. The traditional view holds that excessive accumulation of metals directly damages the islets to induce GDM. Recent studies suggest that supplementing with trace elements can alleviate insulin resistance by reducing endoplasmic reticulum stress, indicating that endoplasmic reticulum stress might have a comparable impact in metal-induced GDM. Selenoprotein S (SEPS1), a key regulator of endoplasmic reticulum function, may play a role. This study aims to investigate the interaction between the genetic predisposition of the SEPS1 gene and exposure to metals on GDM from the perspective of endoplasmic reticulum stress. A total of 278 pregnant women with GDM and 278 matched pregnant women without GDM were recruited. Maternal blood samples were collected before delivery to genotype the SEPS1 gene and measure metal levels. We found that rs28533324 and rs894317 were associated with the risk of GDM. The level of chromium (Cr) Q3 in maternal blood increased the risk of GDM, and the level of nickel (Ni) Q4 decreased the risk of GDM. Furthermore, the Q3 and Q4 Cr exhibit a multiplicative interaction with rs894317 in both the codominant and dominant models, and a multiplicative interaction between the Cr Q3 and rs894317 is also observed in the allele model. We found a novel link between SEPS1 gene variation and GDM, with these associations potentially being modified by Cr exposure. Our findings provide new etiological insights into GDM induced by Cr through endoplasmic reticulum stress.