Determining the characteristics of Arctic proglacial lakes is essential for understanding their current and future influence on glacier mass loss, capacity as a carbon sink and the associated impacts for downstream hydrology and ecology. Field observations of how proglacial lake properties influence rates of glacier mass loss remain sparse yet are increasingly critical for accurate projection of lake-terminating glacier responses to warming air and lake temperatures, particularly in high-latitude Scandinavia under the influence of Arctic amplification. Here we combine satellite and field observations of Kaskasapakte Glacier (KG) (a lake-terminating glacier in Arctic Sweden) to reveal the interplay between lake parameters and glacier mass loss from 2008 to 2019. We present the first field evidence of warmer-than-expected water temperatures (> 4 °C at the ice front) at a Scandinavian proglacial lake and illustrate how these drove rapid thermo-erosional undercutting and calving at the terminus, with width-averaged retreat rates of up to 25 m yr⁻¹ and frontal ablation accounting for ∼ 30 % of glacier volume loss between 2015 and 2019.