This study presents a paleoenvironmental and paleoclimatic reconstruction of western subarctic Québec based on the watershed geomorphology of Lake Kaapumticumac (including terraces, stratigraphy, and peatlands) as well as lake sediments, with reference to stratigraphy, grain size, and organic matter content. The integration of data from diverse sources provides valuable information about the regression of the Tyrrell Sea, lake isolation, and lake level fluctuations. Marine processes dominated prior to ca. 6960 cal yr BP, while the marine-lacustrine transition lasted about 500 years (ca. 6960–6400 cal yr BP). In comparison to other study sites in the Whapmagoostui-Kuujjuarapik area, the earlier isolation of Lake Kaapumticumac is consistent with its higher elevation and its greater distance from Hudson Bay. After 6400 cal yr BP, lake evolution was primarily influenced by the climate. Two major climatic periods were recorded: first, the Hypsithermal (ca. 6400–3500 cal yr BP), during which warm conditions caused partial terrestrialization of Lake Kaapumticumac; and second, the post-Hypsithermal or Neoglacial (ca. 3500 cal yr BP to ca. 200 cal yr BP), which triggered the rise in lake levels and caused levees to form in several places around the lake.