Stratospheric sulfate geoengineering could impact the terrestrial carbon cycle by enhancing the carbon sink. With an 8 Tg yr⁻¹ injection of SO₂ to produce a stratospheric aerosol cloud to balance anthropogenic radiative forcing from the Representative Concentration Pathway 6.0 (RCP6.0) scenario, we conducted climate model simulations with the Community Earth System Model – the Community Atmospheric Model 4 fully coupled to tropospheric and stratospheric chemistry (CAM4–chem). During the geoengineering period, as compared to RCP6.0, land-averaged downward visible (300–700 nm) diffuse radiation increased 3.2 W m⁻² (11 %). The enhanced diffuse radiation combined with the cooling increased plant photosynthesis by 0.07 ± 0.02 µmol C m⁻² s⁻¹, which could contribute to an additional 3.8 ± 1.1 Gt C yr⁻¹ global gross primary productivity without explicit nutrient limitation. This increase could potentially increase the land carbon sink. Suppressed plant and soil respiration due to the cooling would reduce natural land carbon emission and therefore further enhance the terrestrial carbon sink during the geoengineering period. This potentially beneficial impact of stratospheric sulfate geoengineering would need to be balanced by a large number of potential risks in any future decisions about the implementation of geoengineering.