<p>Despite growing evidence that heatwaves and physical mixing jointly regulate phytoplankton dynamics, their interactive effects during early-season warming events remain poorly understood. To address this gap, we conducted a mesocosm experiment simulating a short-term spring heatwave under contrasting mixing regimes. Our results showed that phytoplankton growth rates declined sharply during the first two days of the heatwave (mean: − 0.38&#xa0;d⁻¹), indicating that acute thermal stress imposed immediate constraints on population growth. In the absence of mixing, heatwave conditions led to a sustained decline in cell concentration. In contrast, physical mixing significantly alleviated this suppression, increasing growth rates over the 0–6&#xa0;day period from − 0.33 to − 0.12&#xa0;d⁻¹. Multivariate regression further showed that the accumulation of total biovolume was primarily promoted by phosphorus availability under intermediate mixing conditions, while being negatively related to surface warming intensity. These findings suggest that physical mixing acts as a critical buffering mechanism against heatwave-induced suppression of phytoplankton growth by weakening thermal stratification and promoting vertical nutrient exchange. Overall, this study highlights the importance of considering interactions between physical mixing and thermal stress, particularly within a seasonal context, when assessing ecosystem responses to increasingly frequent heatwaves under climate change.</p>