Westcliff-on-Sea is a town in southeast Essex, England, within the administrative boundaries of the Borough of Southend-on-Sea. It is at the mouth of the Thames Estuary, on its north side, and about 34 miles (54.7 km) east of London.

The London, Tilbury and Southend Railway line arrived in the 1880s, connecting the town with London and shortening travel time to and from that city. The Westcliff railway station in Station Road is now managed by c2c.

Primarily a seaside resort, the shingle and pebbles along the stretch of beach have been covered with imported sand. A notable landmark is the 1.3 mile (2 km) Southend pier, lying just to the east of Westcliff-on-Sea, which extends out into the Estuary and is the longest pleasure pier in the world.

Along the top of the cliffs are spectacular views over the Thames Estuary towards the distant Kent coastline. Not far from the Royal Mews are the beautiful Prittlewell Square gardens and farther along is the Beecroft Art Gallery in Station Road, which is just opposite the famous Cliffs Pavilion and its variety acts. In London Road is the Palace Theatre and a wide range of restaurants, while the New Empire Theatre is in Alexandra Street. The main shopping area in Westcliff-on-Sea is Hamlet Court Road, with many boutiques, antique shops and speciality stores.

Westcliff-on-Sea is home to the Thames Estuary Yacht Club and the Westcliff Casino, which is the largest provincial casino and the second largest in Great Britain.

The British philosopher, the late Professor Sir Bernard Williams, was born in Westcliff-on-Sea.