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Sugar Will Eventually be Exported Transporter (SWEET) proteins are key transporters in sugar transportation. They are involved in the regulation of plant growth and development, hormone crosstalk, and biotic and abiotic stress responses. However, SWEET family genes have not been explored in the sweet potato. In this study, we identified 27, 27, and 25 SWEETs in cultivated hexaploid sweet potato (<i>Ipomoea batatas</i>, 2n = 6x = 90) and its two diploid relatives, <i>Ipomoea trifida</i> (2n = 2x = 30) and <i>Ipomoea triloba</i> (2n = 2x = 30), respectively. These SWEETs were divided into four subgroups according to their phylogenetic relationships with <i>Arabidopsis</i>. The protein physiological properties, chromosome localization, phylogenetic relationships, gene structures, promoter <i>cis</i>-elements, protein interaction networks, and expression patterns of these 79 <i>SWEETs</i> were systematically investigated. The results suggested that homologous SWEETs are differentiated in sweet potato and its two diploid relatives and play various vital roles in plant growth, tuberous root development, carotenoid accumulation, hormone crosstalk, and abiotic stress response. This work provides a comprehensive comparison and furthers our understanding of the SWEET genes in the sweet potato and its two diploid relatives, thereby supplying a theoretical foundation for their functional study and further facilitating the molecular breeding of sweet potato.