Pin Mill is a hamlet on the south bank of the tidal River Orwell, on the outskirts of the village of Chelmondiston, on the Shotley peninsula in southern Suffolk. It lies within the Suffolk Coast and Heaths Area of Outstanding Natural Beauty and is a designated Conservation Area. It is now generally known for the historic Butt and Oyster public house, and for sailing.

The River Orwell flows through the county of Suffolk in England from Ipswich to Felixstowe. Above Ipswich, the river is known as the River Gipping, but its name changes to the Orwell at Stoke Bridge, where the river becomes tidal. It broadens into an estuary at Ipswich, where the Ipswich dock has operated since the 7th century, and then flows into the North Sea at Felixstowe, the UK's largest container port, after joining the River Stour at Shotley forming Harwich harbour.