Crowninshield Island, also known as Brown's Island, is a small island off the shore of Marblehead, Massachusetts. Since 1955, it has been maintained by The Trustees of Reservations. For all Trustees of Reservations purposes, the island is referred to as Crowninshield Island, however federally the island is still maintained as "Brown's Island", and it remains better known by that name locally.

The island had been owned by the Brown family from 1797-1918. The Massachusetts Trustees of Reservations describes themselves as "the oldest statewide land conservation organization in the country." In honor of Louise E. du Pont Crowninshield, a historic preservationist who married Francis Boardman Crowninshield, a member of the noted Crowninshield family, they suggested to call it Crowninshield Island, honoring the pioneer in America's historic preservation movement from 1925 until her death in 1958. Crowninshield summered at Peach's Point and the family donated the island to the trustees in 1955. The Crowninshields purchased the island from the Browns for $1 in 1918. The Crowninshields were Boston Brahmins who had been historically powerful in nearby Salem, the original town from which Marblehead was formed.