The question of how saline–alkaline groundwater can be used as a CO₂ sink in arid saline–alkaline areas remains controversial. This study investigates the role of saline–alkaline groundwater as a CO₂ sink using a mass balance method, Gibbs diagrams of the hydrochemistry, and carbon isotope (δ¹³C_DIC) measurements. Twenty-eight groundwater samples of varying electrical conductivity (EC; 1.52–52.34 mS cm⁻¹) were collected at different depths (1–2 and 5–25 m) in the Hetao Basin of Inner Mongolia, China. The results showed that groundwater ions could be primarily concentrated from water–rock interactions and evaporation, and that there are two main groundwater types: a mixed Na·Ca·Mg–Cl·SO₄·HCO₃ type and a Na–Cl type. The dissolved inorganic carbon (DIC) concentration in samples obtained from a depth of 1–2 m was less than that in samples from 5–25 m, and a downward migration trend of DIC in the groundwater was observed. The DIC concentration exhibited a significant positive correlation with pH (R² = 0.61, p < 0.05) and the saturation index of carbonates (R² = 0.93, p < 0.01). Groundwater with a higher pH contained a higher DIC concentration and could provide strong carbon sink potential. The δ¹³C_DIC values of the groundwater samples varied from −21.22‰ to −11.02‰, indicating that DIC was derived from the dissolution equilibrium of pedogenic carbonates and atmospheric/soil CO₂. The carbon sequestration of the shallow saline–alkaline groundwater in the Hetao Basin could reach 4.66 × 10⁸ g C a⁻¹, which represents important potential of carbon sink in the biogeochemical cycle.