Arctic warming is expected to increase thermokarst erosion in thaw lakes, thus inducing large emissions of CH₄ to the atmosphere. To reduce uncertainties about the mechanisms, magnitude and timing of methane emissions, we conducted an in-situ experiment to simulate lake expansion following thermokarst erosion. Three tundra horizons were excavated, incubated at the bottom of a thaw lake and subsequently monitored for CH₄ ebullition from mid-season until lake freeze-up. Although the permafrost and seasonally frozen active layer emitted little CH₄ throughout the experiment, ebullition from the thawed active layer began the first week, quickly reaching rates (39 mg CH₄ m⁻² day⁻¹) comparable to background ebullition in Siberian thaw lakes. While the long-term fate of permafrost carbon from thermokarst lake expansion remains uncertain, the short-term methane release is dominated by upper active layer carbon. Thus, deepening of the upper active layer in a thaw lake dominated landscape should increase CH₄ ebullition rates.