Natural harbour on the southeast coast of the Isle of Man, immediately east of Ronaldsway and Castletown.

Derbyhaven was known in earlier forms as Rainsway or Ronaldsway and served as the principal roadstead for Castletown. Sacheverell described it as "a reasonable good harbour", secured by a fort on a small island dedicated to St Michael.

St Michael's Isle lies at the eastern side of the bay and carried both a chapel and fortification. The harbour and island formed part of the defensive approaches to Castletown, while the broader bay gave vessels more room than the small inner harbour beside Castle Rushen.

The shore around Ronaldsway and Derbyhaven was also the scene of medieval fighting. In 1270 a Scottish force landed there and defeated the Manx. Later accounts give the Manx loss as Ivar and 686 of the chief men of the Island.

Richard de Mandeville and his Irish followers landed at Ronaldsway in 1316 before advancing inland and defeating the Manx near South Barrule.

In 1650 James Stanley, seventh Earl of Derby, narrowly escaped an attack in the bay. As he returned by boat from visiting a Parliamentary vessel, a gun was fired at him. Colonel Snaid, Colonel Richard Weston and Philip Lucas, the master of the boat, were killed.

Robertson's list of appointed ports in 1776 names Derbyhaven as a port with Castletown harbour, Poolvash, Port le Marie, Port Erin, Fleshwick and Niarbyl as its creeks.