Here we present the first tree-ring series (1850–2003) of stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope ratios from a high-latitude treeline site in northwestern Canada. Both δ¹³C and δ¹⁸O were measured at annual resolution from whole-ring α-cellulose of three white spruce trees (Picea glauca [Moench] Voss) growing in the Mackenzie Delta. There is a strong positive association between δ¹³C and maximum summer temperatures. This relation likely results from the influence of temperature-induced drought stress on stomatal conductance. Mean summer relative humidity is also significantly correlated, inversely, with δ¹³C reflecting its direct influence on stomatal conductance. The δ¹⁸O record is strongly and positively correlated with early-spring to mid-summer minimum temperatures likely owing to the temperature dependence of δ¹⁸O in precipitation and uptake of this water during the growing season. Mean summer relative humidity is also significantly and inversely correlated with δ¹⁸O due to leaf water evaporative enrichment. Our δ¹³C and δ¹⁸O records contain a large amount of climate-driven variability indicating their considerable potential to infer past climate changes in the Mackenzie Delta region.