Southend-on-Sea, borough and unitary authority (1991 pop. 155,720), E England, at the mouth of the Thames River. The city is a popular seaside resort; its extensive amusement pier suffered major damage from fire in 2005. Manufactures include aluminum foil, paint, electronic equipment, and pharmaceuticals. Yachting events are held there annually. Southend-on-Sea also has an airport.