Abstract. Soil carbon stored in high-latitude permafrost landscapes is threatened by warming and could contribute significant amounts of carbon to the atmosphere and hydrosphere as permafrost thaws. Thermokarst and permafrost disturbances, especially active layer detachments and retrogressive thaw slumps, are present across the Fosheim Peninsula, Ellesmere Island, Canada. To determine the effects of retrogressive thaw slumps on net ecosystem exchange (NEE) of CO₂ in high Arctic tundra, we used two eddy covariance (EC) tower systems to simultaneously and continuously measure CO₂ fluxes from a disturbed site and the surrounding undisturbed tundra. During the 32-day measurement period in the 2014 growing season, the undisturbed tundra was a small net sink (NEE  =  −0.1 g C m⁻² d⁻¹); however, the disturbed terrain of the retrogressive thaw slump was a net source (NEE  =  +0.4 g C m⁻² d⁻¹). Over the measurement period, the undisturbed tundra sequestered 3.8 g C m⁻², while the disturbed tundra released 12.5 g C m⁻². Before full leaf-out in early July, the undisturbed tundra was a small source of CO₂ but shifted to a sink for the remainder of the sampling season (July), whereas the disturbed tundra remained a source of CO₂ throughout the season. A static chamber system was also used to measure daytime fluxes in the footprints of the two towers, in both disturbed and undisturbed tundra, and fluxes were partitioned into ecosystem respiration (R_e) and gross primary production (GPP). Average GPP and R_e found in disturbed tundra were smaller (+0.40 µmol m⁻² s⁻¹ and +0.55 µmol m⁻² s⁻¹, respectively) than those found in undisturbed tundra (+1.19 µmol m⁻² s⁻¹ and +1.04 µmol m⁻² s⁻¹, respectively). Our measurements indicated clearly that the permafrost disturbance changed the high Arctic tundra system from a sink to a source for CO₂ during the majority of the growing season (late June and July).