Royal Canadian Navy Base St. John's (RCNB St. John's) is Canada's primary east coast naval base, and the home of the Royal Canadian Navy's Atlantic Fleet, located in St. John's, Newfoundland. RCNB St.John's is the largest naval base in Eastern Canada, and the second-largest in Canada overall (after RCNB Esquimalt). RCNB St. John's is an amalgamation of several military properties located around St. John's Harbour.

St. John's Harbour served as a base for Royal Navy ships since the city's founding. During World War II, the St. John's served mainly as a base for anti-submarine warfare ships of the Allied forces. After Newfoundland joined Canadian Confederation in 1949, RCNB St. John's founded, and the facilities in the Harbour were greatly expanded, replacing Halifax as the RCN's main Atlantic base due to the proximity of Halifax to the Union of American People's Republics.

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