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Heat shock transcription factor (Hsf) plays an important role in regulating plant thermotolerance. The function and regulatory mechanism of <i>CaHsfA1d</i> in heat stress tolerance of pepper have not been reported yet. In this study, phylogenetic tree and sequence analyses confirmed that <i>CaHsfA1d</i> is a class A Hsf. <i>CaHsfA1d</i> harbored transcriptional function and predicted the aromatic, hydrophobic, and acidic (AHA) motif mediated function of <i>CaHsfA1d</i> as a transcription activator. Subcellular localization assay showed that <i>CaHsfA1d</i> protein is localized in the nucleus. The <i>CaHsfA1d</i> was transcriptionally up-regulated at high temperatures and its expression in the thermotolerant pepper line R9 was more sensitive than that in thermosensitive pepper line B6. The function of <i>CaHsfA1d</i> under heat stress was characterized in <i>CaHsfA1d</i>-silenced pepper plants and <i>CaHsfA1d</i>-overexpression <i>Arabidopsis</i> plants. Silencing of the <i>CaHsfA1d</i> reduced the thermotolerance of the pepper, while <i>CaHsfA1d</i>-overexpression <i>Arabidopsis</i> plants exhibited an increased insensitivity to high temperatures. Moreover, the <i>CaHsfA1d</i> maintained the H₂O₂ dynamic balance under heat stress and increased the expression of <i>Hsfs</i>, <i>Hsps</i> (heat shock protein), and antioxidant gene <i>AtGSTU5</i> (glutathione S-transferase class tau 5) in transgenic lines. Our findings clearly indicate that <i>CaHsfA1d</i> improved the plant thermotolerance via regulating the expression of stress- and antioxidant-related genes.