The ice-on and ice-off dates, the duration of the ice-covered period, spring and autumn mixing, and summer stratification were determined based on water temperature measurements at an autonomous station in the central deep-water part of Petrozavodsk Bay of Lake Onego in 2023–2024. Deviations were revealed in the stages of the annual thermal cycle from the average long-term values: ice-on occurred were earlier in the cold autumn months, ice break-up happened earlier in the warm spring months, the spring thermal bar passed 10–15 days earlier than historically, and the duration of summer stratification was a month longer. Due to the abnormally warm weather in August and September in these years (the average monthly air temperature in August of both years was 1.7 °C higher than the 1991–2020 baseline, and in September 2023 and 2024 it was 4.0 and 5.3 °C higher, respectively), the biological summer lasted 2–3 weeks longer than normally. In the summer of 2023 and 2024, repeated episodes of a sharp rise in the temperature of the bottom water layer with a subsequent return to the background values were observed at the measuring station. The descent of warm water from the surface layer could occur during the convergence of currents. Periods with abnormally high bottom water temperatures (5–10 °С higher than typical values) in the summer of 2024 lasted for more than 1.5 months, which could have a negative impact on the benthic community adapted to colder conditions. The identified changes in the thermal and ice regimes of the lake may affect the functioning of the lake ecosystem, which requires monitoring arrangements to be made for the future.