REIC (reduced expression in immortalized cells) has been identified as a gene whose expression was reduced in immortalized cultured cells. The REIC gene is identical to Dickkopf-3 (Dkk3), which encodes a secreted glycoprotein belonging to the Dkk family. Previously, we showed that Dkk3 protein is present in the mouse adrenal medulla. However, its role in this tissue has not been elucidated. To explore it, we performed electron microscopic (EM) studies and RNA-sequencing (RNA-seq) analysis on Dkk3-null adrenal glands. EM studies showed that the number of dense core secretory vesicles were significantly reduced and empty vesicles were increased in the medulla endocrine cells. Quantitative PCR (qPCR) analysis showed relative expression levels of chromogranin A (Chga) and neuropeptide Y (Npy) were slightly but significantly reduced in the Dkk3-null adrenal glands. From the result of RNA-seq analysis as a parallel study, we selected three of the downregulated genes, uncoupled protein-1 (Ucp1), growth arrest and DNA-damage-inducible 45 gamma (Gadd45g), and Junb with regard to the estimated expression levels. In situ hybridization confirmed that these genes were regionally expressed in the adrenal gland. However, expression levels of these three genes were not consistent as revealed by qPCR. Thus, Dkk3 maintains the integrity of secreting vesicles in mouse adrenal medulla by regulating the expression of Chga and Npy.