An ion channel is a pore-forming protein that helps to establish and control the small voltage gradient across the plasma membrane of all living cells (see resting potential) by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells.

An ion channel is an integral membrane protein or more typically an assembly of several proteins.

The 2003 Nobel Prize in Chemistry was awarded to Peter Agre and Roderick MacKinnon for their discovery of ion channels.