Castlemaine Harbour is a Ramsar site, Special Area of Conservation, Special Protection Area and national nature reserve of approximately 2,281 acres (9.23 km2) located in County Kerry, Ireland.

Castlemaine Harbour was legally protected as a national nature reserve by the Irish government in 1990.

The reserve includes salt marshes, sandbanks, and mudflats which are protected from the sea by a large system of dunes.

One of the four largest Zostera beds in Ireland is found in the Harbour's mudflats, and these provide food for a wide variety of water birds which overwinter there, including the light-bellied Brent goose, Branta bernicla hrota.

[2][4] Among the birds recorded on the site are sanderlings, oystercatchers, red-throated divers, and greenshanks.