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The Bcl-2-associated athanogene (BAG) proteins are a family of multi-functional group of co-chaperones regulators, modulating diverse processes from plant growth and development to stress response. Here, 10 members of <i>SlBAG</i> gene family were identified based on the available tomato (<i>Solanum lycopersicum</i>) genomic information and named as <i>SlBAG1-10</i> according to their chromosomal location. All SlBAG proteins harbor a characteristic BAG domain, categorized into two groups, and SlBAG4, SlBAG7, and SlBAG9 of group I contain a plant-specific isoleucine glutamine (IQ) calmodulin-binding motif located in the N terminus. The quantitative real-time PCR expression analysis revealed that these <i>SlBAG</i> genes had organ-specific expression patterns and most <i>SlBAG</i> genes were differentially expressed in multiple abiotic stresses including drought, salt, high temperature, cold, and cadmium stress as well as abscisic acid and H₂O₂. In addition, heterologous overexpression of <i>SlBAG9</i> increased the sensitivity of Arabidopsis to drought, salt, and ABA during seed germination and seedling growth. The decreased tolerance may be due to the downregulation of stress-related genes expression and severe oxidative stress. The expression levels of some stress and ABA-related genes, such as <i>ABI3</i>, <i>RD29A</i>, <i>DREB2A</i>, and <i>P5CS1</i>, were significantly inhibited by <i>SlBAG9</i> overexpression under osmotic stress. Meanwhile, the overexpression of <i>SlBAG9</i> inhibited the expression of <i>FSD1</i> and <i>CAT1</i> under stress conditions and the decreased levels of superoxide dismutase and catalase enzyme activities were detected accompanying the trends in the expression of both genes, which resulted in H₂O₂ accumulation and lipid peroxidation. Taken together, these findings lay a foundation for the future study of the biological function of <i>SlBAG</i> genes in tomato.