A practical route from oxetane or thietane to γ‐(thio)butyrolactone via solvated‐proton‐assisted cobalt‐catalyzed carbonylative ring expansion under syngas atmosphere has been established. A wide variety of γ‐(thio)butyrolactones can be afforded in good to excellent yields. The versatility of this method has been well demonstrated in the synthesis of intermediates towards the natural product Arctigenin