Snail family proteins regulate transcription of molecules for cell–cell adhesion during epithelial–mesenchymal transition (EMT). Based on putative glycogen synthase kinase 3β (GSK‐3β) phosphorylation sites within the Slug/Snail2, we explored the significance of GSK‐3β‐mediated phosphorylation in Slug/Snail2 expression during EMT. Mutation of the putative GSK‐3β phosphorylation sites (S92/96A or S100/104A) enhanced the Slug/Snail2‐mediated EMT properties of E‐cadherin repression and vimentin induction, compared with wild‐type Slug/Snail2. S92/96A mutation inhibited degradation of Slug/Snail2 and S100/104A mutation extended nuclear stabilization. Inhibition of GSK‐3β activity caused similar effects, as did the phosphorylation mutations. Thus, our study suggests that GSK‐3β‐mediated phosphorylation of Slug/Snail2 controls its turnover and localization during EMT.