Ecological and biogeochemical processes in lakes are strongly dependent upon water temperature. Long-term surface warming of many lakes is unequivocal, but little is known about the comparative magnitude of temperature variation at diel timescales, due to a lack of appropriately resolved data. Here we quantify the pattern and magnitude of diel temperature variability of surface waters using high-frequency data from 100 lakes. We show that the near-surface diel temperature range can be substantial in summer relative to long-term change and, for lakes smaller than 3 km2, increases sharply and predictably with decreasing lake area. Most small lakes included in this study experience average summer diel ranges in their near-surface temperatures of between 4 and 7°C. Large diel temperature fluctuations in the majority of lakes undoubtedly influence their structure, function and role in biogeochemical cycles, but the full implications remain largely unexplored. ; Peer-reviewed. Copyright: © 2016 Woolway et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.PLoS ONE 11(3): e0152466. doi:10.1371/journal.pone.0152466 ; This project was funded by OTT Hydrometry Ltd and a University College London impact award awarded to RIW and is part of the Natural Environment Research Council (NERC) Sensor Network project United Kingdom Lake Ecological Observatory Network (UKLEON;NE/I007407/1). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.