The Aux/IAA (auxin/indole-3-acetic acid) gene family coordinates many key processes in the adaptive growth of plants. However, limited information is available about the regulatory mechanism of SlIAA9 (Solyc04g076850), which is a member of the Aux/IAAs. The goal of this study was to analyze the pattern of expression and regulatory mechanism of SlIAA9 in detail. The promoter of SlIAA9 gene with 1600 bp upstream of ATG was cloned from tomato (Solanum lycopersicum cv. Ailsa Craig). The promoter analysis suggests that abiotic stressors and phytohormones may stimulate the expression of SlIAA9. Furthermore, we confirmed that the SlIAA9 transcript was induced by auxin, gibberellin, salicylic acid, abscisic acid, methyl jasmonate, heat, cold, drought, and salt stress. We preliminarily found that SlIAA9 plays a negative role in the drought signaling response by affecting the ability to scavenge reactive oxygen species. To analyze the function of the SlIAA9 promoter in more detail, we created the proSlIAA9:GUS transgenic tomato line. GUS staining showed that the SlIAA9 promoter was highly active in the roots, leaves, sepals, petals and anthers of tomato seedlings. The promoter was expressed the most highly in the flowers. To further verify the activity of the SlIAA9 promoter, we found that different segments of the SlIAA9 promoter had different regulatory effects on the expression of downstream genes in response to IAA. A 400 bp fragment was identified as the key functional region for IAA response.