Groundwater discharge to headwater streams and concomitant terrestrial dissolved inorganic carbon (DIC) export play a significant role in headwater stream CO2 evasion. However, previous studies rarely examined diffuse groundwater discharge and its impact on headwater stream CO2 evasion, thereby lacking the understanding of the role of diffuse groundwater discharge in terrestrial DIC export and stream CO2 evasion. This study quantified diffuse groundwater discharge along a 43 km semiarid headwater stream by combining hydraulic, isotopic (radon-222) and chemical (electrical conductivity) approaches, and estimated the reach-level CO2 budgets of the stream. Reach-scale water and mass balance modeling yielded highly variable diffuse groundwater discharge rates (n = 16, range: 1.08-7.80 m2/d, mean ± 1 sd: 4.57 ± 1.81 m2/d). Groundwater was supersaturated with CO2 at all sites, with strongly variable CO2 partial pressure (pCO2) and DIC concentrations at 1,223-27,349 μatm and 30-119 mg/L, respectively. Diffuse groundwater discharge dominated terrestrial DIC export to the stream (12-111 g C m-2 d-1, normalized to water surface area). A portion of groundwater dissolved CO2 transported to the stream was emitted to the atmosphere with evasion rates varying at 0.62-3.18 g C m-2 d-1. However, most dissolved CO2 was transformed into HCO3- through carbonate buffering because of the regulation of carbonate equilibrium. Overall, the stream CO2 evasion was driven by carbon transfer but limited by carbon supply. This study provides a bottom-up perspective to understand terrestrial DIC export and stream CO2 evasion in arid and semiarid areas.