Ischaemic stroke (IS) has a heavy disease burden. Although epidemiological research has suggested that heatwaves are associated with cardiovascular disease, there is a lack of empirical evidence for a correlation between heatwaves and IS. The China Meteorological Administration defines a heatwave as a wave lasting ≥ 3 days, with a maximum temperature of ≥ 35℃. We collected data on daily meteorological conditions, air pollution, and IS admissions in Hunan Province from 2018 to 2019. A generalized additive model and distributed lag nonlinear model were used to determine the associations between heatwaves and IS admissions. We analysed 329,876 admitted patients with IS in Hunan Province from 2018 to 2019. The main effect of heatwaves was that they significantly increased the risk of hospitalization for IS. The single-day lag maximum risk occurred at a daily average temperature of 30.88℃ (RR = 1.05, 95% CI: 1.04–1.06) and at a daily maximum temperature of 35.82℃ (RR = 1.05, 95% CI: 1.03–1.06). The use of the 5th and subsequent days of a heatwave as a reference showed that the 1st–2nd days (RR = 1.07, 95% CI: 1.02–1.12) and the 3rd–4th days (RR = 1.68, 95% CI: 1.03–1.10) of the heatwave increased the risk of hospitalization for IS. Compared with the third and subsequent heatwaves, the first (RR = 1.27, 95% CI: 1.19–1.35) and second (RR = 1.24, 95% CI: 1.16–1.32) heatwaves had greater impacts on the risk of hospitalization for IS. The risk of IS hospitalization was also exacerbated by high relative humidity (RR = 1.25, 95% CI: 1.16–1.35) and a low diurnal temperature range (RR = 1.08, 95% CI: 1.02–1.14) during the heatwave period. In our study, the main effects of heatwaves increased the risk of IS hospitalization. The effects varied according to the day of the heatwave, the timing of the heatwave, the DTR during the heatwave, and the humidity during the heatwave. This evidence has significant implications for the strategic planning of public health interventions to mitigate adverse health outcomes associated with heatwaves.