The Cypress Hills of southwestern Saskatchewan and southeastern Alberta are a key area for Quaternary research because the permanent lakes and coniferous forest are sources of proxy paleoenvironmental data generally not available on the surrounding northern Great Plains. In this paper, the Holocene paleoclimate of the western Cypress Hills is reconstructed from the pollen and organic matter contents of a 9.6 m sediment core from Harris Lake, Saskatchewan. This climatic history is considerably longer than others from the semiarid Interior Plains of Canada. Chronological control consists of a basal conventional radiocarbon date of 9120 ± 250 BP, Mazama tephra, and three accelerator mass spectrometry dates from plant macrofossils. The data from Harris Lake indicate significant climatic change from the warm, dry altithermal, 7700–5100 BP, to cooler and moister conditions of the late Holocene. Low lake sediment organic matter from 3000 to 2400 BP is attributed to lower temperatures and coincides with a well-documented global neoglacial advance. Correlations among the proposed climatic history, sedimentation rates in Harris Lake, and dated soils and sediments described in the literature are the basis for a reconstruction of the Holocene geomorphology. This information suggests a transition in dominant geomorphic processes from fluvial and aeolian erosion before 5100 BP to rotational landsliding after 4500 BP.