Middleton-on-Sea is a village, civil parish and an electoral ward in the Arun District of West Sussex, England, lying to the east of Bognor Regis and neighbouring Felpham. The parish also contains the settlements of Elmer and Ancton. The southern half is urban and the northern rural.

The village has a pub, The Beresford Arms, named after Viscount William Beresford, who was a general in the British Army, and also commander of the Portuguese Armed Forces.

In the 2001 census, 5,105 people lived in 2,366 households, of whom 2,206 were economically active, a lower than average proportion but higher than other coastal resorts. The 2011 population was 5,077.

Middleton was listed in the Domesday Book of 1086 as having 16 households and a church.

The ancient parish called Middleton had 370 acres (150 ha) in 1881 but had been reduced in area in previous centuries by sea erosion. The configuration of the western and northern boundaries suggests that the parish was once part of Felpham, and the name Middleton may refer to the manor's central position between Felpham and either Elmer or Cudlow in Clymping. Part of the eastern boundary was formed c. 1310 by a ditch and part by Elmer pool, while the north-eastern boundary follows the Ryebank rife. The parish was enlarged in 1933 by the addition of Ancton from Felpham parish and in 1971 had 892 acres (361 ha); its name was extended in 1934 to prevent confusion with other Middletons.

In 1801 there were only six houses in the parish; in 1841 there were nineteen, including the new manor house at Middleton and newly built coastguard cottages at Elmer. Most of the latter remained unoccupied after the departure of the coastguard until demolition between 1910 and 1932. Two larger houses were built during the same period: Middleton Field west of Yapton Road, home of the owner of the adjacent brickworks, and the half-timbered and pebbledashed Ancton Lodge.

At the junction of Middleton and Elmer roads with Yapton Road in 1606 were two or three dwellings; only one older building remained there in 1996, the one-storeyed, flint and thatched Rose Cottage, which is apparently 17th-century or earlier with 19th-century additions.

In 1910 the engineer Norman Thompson, attracted to the area by the large expanse of firm sand and the constant winds along the shore, founded an aircraft works which after the removal of much of the sand in a storm in 1913 turned to making seaplanes. The firm was later called the Norman Thompson Flight Co. During the First World War it supplied aircraft to the navy, the workforce growing from ten at the beginning to between 700 and 900. About 250 aircraft in all were built, but with the cancellation of orders at the end of the war the firm went into liquidation.