In S. exigua, ingestion of Cry1Ac reduces larval growth, shortens lifespan, and decreases copulation and oviposition of the adults. Cadherin‐like protein SeCad1b in S. exigua has recently been published. Here, we tested whether SeCad1b mediates the negative effects of Cry1Ac. We identified three potential Cry toxin binding regions in SeCad1b, i.e., 879EIAIQITDTNN889, 1357SLLTVTI1363, and 1436GVISLNFQ1443. We expressed and purified a truncated cadherin, rSeCad1bp, and its interspecific homologue, rHaBtRp, from H. armigera that contain the putative toxin binding regions. Using a toxin overlay assay, we found that rSeCad1bp specifically binds to biotinylated Cry1Ac in a dose‐dependent manner. We also discovered that an addition of rSeCad1bp and rHaBtRp enhances the suppression of larval growth by Cry1Ac, although rSeCad1bp is less suppressive than rHaBtRp. Finally, RNA interference‐mediated knockdown of SeCad1b reduced approximately 80% of the target gene and significantly alleviated the negative effect of CrylAc on larval growth. We infer that the  S. exigua SeCad1b is a functional receptor of Cry1Ac.