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The <i>BES1</i> (BRI1-EMSSUPPRESSOR1) gene family is a unique class of transcription factors that play dynamic roles in the Brassinosteroids (BRs) signaling pathway. The published genome sequences of a large number of plants provide an opportunity to identify and perform a comprehensive functional study on the <i>BES1</i> gene family for their potential roles in developmental processes and stress responses. A total of 135 <i>BES1</i> genes in 27 plant species were recognized and characterized, which were divided into five well-conserved subfamilies. <i>BES1</i> was not found in lower plants, such as <i>Cyanophora paradoxa</i> and <i>Galdieria sulphuraria</i>. The spatial expression profiles of <i>BES1s</i> in Arabidopsis, rice, and cotton, as well as their response to abiotic stresses, were analyzed. The overexpression of two rice <i>BES1</i> genes, i.e., <i>OsBES1-3</i> and <i>OsBES1-5</i>, promotes root growth under drought stress. The overexpression of <i>GhBES1-4</i> from cotton enhanced the salt tolerance in Arabidopsis. Five protein interaction networks were constructed and numerous genes co-expressed with <i>GhBES1-4</i> were characterized in transgenic Arabidopsis. <i>BES1</i> may have evolved in the ancestors of the first land plants following its divergence from algae. Our results lay the foundation for understanding the complex mechanisms of <i>BES1</i>-mediated developmental processes and abiotic stress tolerance.