Newark Castle is a well-preserved castle sited on the south shore of the estuary of the River Clyde in Port Glasgow, Inverclyde, Scotland, where the firth gradually narrows from the Firth of Clyde and navigation upriver is made difficult by shifting sandbanks. For centuries this location was used to offload seagoing ships, and led to the growth of Port Glasgow close to the castle on either side and to the south. When dredging techniques made the Clyde navigable as far as Glasgow the port became a shipbuilding centre, and the castle was surrounded by shipyards. Ferguson Shipbuilders, the last shipyard on the lower Clyde, stands close to the west of the castle, but the shipyards to the east were removed around the 1980s and new landscaped areas formed to the east of Newark Castle, opening up scenic views of the castle and across the Clyde from a new bypass road.

Port Glasgow is the second-largest town in the Inverclyde council area of Scotland. The population according to the 1991 census for Port Glasgow was 19,426 persons and in the 2001 census was 16,617 persons. The most recent census in 2011 states that the population has declined to 15,414. It is located immediately to the east of Greenock and was previously a burgh in the county of Renfrewshire.