Seed weight is an important target trait in pomegranate breeding and culture. Expansins act by loosening plant cell walls and cellulosic materials, permitting turgor-driven cell enlargement. However, the role of expansin genes () in pomegranate seed weight remains elusive. A total of 29 were identified in the 'Dabenzi' genome. These genes were classified into four subfamilies and 14 subgroups, including 22 , 5 , 1 , and 1 . Transcriptome analysis of in different tissues (root, leaf, flower, peel, and seed testa) in 'Dabenzi', and the seed testa of the hard-seeded pomegranate cultivar 'Dabenzi' and soft-seeded cultivar 'Tunisia' at three development stages showed that three (, , ) were highly expressed throughout seed development, especially in the sarcotesta. SNP/Indel markers of these were developed and used to genotype 101 pomegranate accessions. The association of polymorphic with seed weight-related traits (100-seed weight, 100-kernel weight, 100-sarcotesta weight, and the percentage of 100-sarcotesta to 100-seed weight) were analyzed. was significantly associated with 100-seed weight and 100-sarcotesta weight and is a likely candidate for regulating seed weight and sarcotesta development in particular. This study provides an effective tool for the genetic improvement of seed weight in pomegranate breeding programs.