Juanita Joan Nielsen (née Smith; 22 April 1937 – disappeared 4 July 1975) was an Australian newspaper publisher, journalist and activist, and heiress of the Mark Foy family. She was notable for her advocacy against the urban development on heritage Victoria Street, Kings Cross, Sydney, and her support for the Builders Labourers' Federation's Green Bans. Nielsen disappeared in Kings Cross in July 1975. The people responsible for her disappearance have never been identified, and her remains have never been found.