Abstract: Global warming and more frequent heatwaves have made high-temperature stress a major constraint on seaweed aquaculture. Caulerpa lentillifera is sensitive to elevated water temperature, resulting in unstable yield and quality. Here, we evaluated whether exogenous naphthaleneacetic acid (NAA) mitigates short-term heat stress. Thalli were pretreated with NAA (0, 50, 100, 200, 400, and 800 μg·L⁻¹) for 14 d under normal conditions and then exposed to acute heat stress for 48 h. NAA effects were dose dependent, showing an overall pattern of stimulation followed by attenuation at higher doses. Heat stress markedly inhibited growth and intensified oxidative damage in the control. The 50 μg·L⁻¹ treatment produced negligible alleviation, whereas 100–200 μg·L⁻¹ improved growth and reduced malondialdehyde while enhancing chlorophyll and antioxidant enzyme activities. The 400 μg·L⁻¹ treatment performed best in maintaining growth, promoting soluble sugar accumulation, increasing catalase activity, and preserving tissue integrity, while 800 μg·L⁻¹ conferred weaker benefits. Principal component analysis identified 400 μg·L⁻¹ as the optimal concentration. Collectively, appropriate NAA application enhances antioxidant capacity, sustains photosynthetic performance and carbon assimilation, and stabilizes tissue structure, thereby improving short-term heat tolerance in Caulerpa lentillifera.