The River Alyn (Welsh: Afon Alun) is a tributary of the River Dee. The river Alyn rises at the southern end of the Clwydian hills and the Alyn Valley forms part of the Clwydian Range Area of Outstanding Natural Beauty. The main town on the River Alyn is Mold, the county town of Flintshire.
The River Alyn crosses the carboniferous limestone from Halkyn Mountain and down through the Loggerheads area before making its confluence near Mold. Throughout the length of the Alyn there are numerous sinkholes and caverns and during the summer months long stretches of the river bed run dry. The caves in the Alyn Gorge between Loggerheads and Rhydymwyn include Ogof Hesp Alyn, Ogof Hen Ffynhonau and Ogof Nadolig.