Global climate warming and extreme high-temperature events significantly impact crop growth, development, and economic productivity. Plants typically enhance heat tolerance by regulating heat shock transcription factors (HSFs) and antioxidant enzymes to reduce the detrimental impacts of heat stress. However, the precise regulatory mechanisms by which HSFs confer heat tolerance in tomato remain to be elucidated. In this study, we investigated the role of the heat-induced transcription factor SlHSFA3 in tomato heat tolerance. We demonstrated that knockout of SlHSFA3 increases tomato plants’ sensitivity to heat stress, while enhanced expression of SlHSFA3 improves heat tolerance by promoting reactive oxygen species (ROS) scavenging through increased ascorbate peroxidase (APX) activity. Similarly, overexpression of SlAPX1 enhances heat tolerance in tomato by reducing ROS accumulation, whereas its knockout increases heat stress sensitivity. Furthermore, SlHSFA3 interacts with SlAPX1 to further augment APX activity. Molecular analysis revealed that SlHSFA3 directly upregulates the expression of SlAPX1 by binding to its promoter region. In summary, our findings elucidate the molecular mechanism of the SlHSFA3-SlAPX1 regulatory pathway in tomato heat tolerance, providing a theoretical foundation for developing advanced biotechnological and breeding strategies to improve crop adaptation under elevated temperature conditions.