Westgate-on-Sea is a seaside town and civil parish on the north-east coast of Kent, England. It is within the Thanet local government district and borders the larger seaside resort of Margate. Its two sandy beaches have remained a popular tourist attraction since the town's development in the 1860s from a small farming community. The town had a population of 7,517 at the 2021 Census.

The town was the location of a Royal Naval Air Service seaplane base at St Mildred's Bay, which defended the Thames Estuary coastal towns during World War I. The town is the subject of Sir John Betjeman's poem, "Westgate-on-Sea". Residents have included the 19th-century surgeon Sir Erasmus Wilson and former archbishop of Canterbury William Temple. The artist Sir William Quiller Orchardson painted several of his best-known pictures whilst living in Westgate-on-Sea. The British artist Louis Wain and his family lived there. The British composer Arnold Cooke attended the town's Streete Preparatory School in the early 20th century, and Eton headmaster Anthony Chenevix-Trench spent the earliest few years of his education in the town.

The town inherited its name from the Westgate Manor, which was located in the area in medieval times. In the early 20th century, the remains of a Roman villa were discovered in what is now Beach Road, where a stream once used to flow. Fresh water can still be seen rising from the sand at low tide.

A Roman villa has been found in fields surrounding Westgate, with strong archaeological evidence of Roman activity between Quex Park East Side in Birchington and the Dent de Lion medieval gatehouse at Garlinge.

Before the 1860s, Westgate consisted of only a farm, a coastguard station (built 1791 and still standing in Old Boundary Road) and a few cottages for the crew that surrounded it. These were located beside the coast at St Mildred's Bay, named after Mildrith, Thanet's patron saint and a one-time Abbess of Minster.

During the late 1860s, businessmen developed the area into a seaside resort for the upper to middle-classes. A stretch of sea wall, with promenade on top, was constructed around the beaches at St Mildred's Bay and West Bay, and the land divided into plots to be sold for what would become an exclusive development by the sea for wealthy metropolitan families within a gated community, rather than for occasional tourists. The opening of a railway station, in 1871, led to the rapid expansion of the population, which reached 2,738 by 1901. The demands of the increasing population led to the building of the parish churches of St. James in 1872 and St. Saviour in 1884. St. Saviour's was designed by the architect C.N. Beazley. In 1884 it was reported that Essex, on the other side of the Thames Estuary, was hit by a tremor so large that it caused the bells of St. James' Church to ring. In 1884, ownership of most of the resort passed to Coutts Bank, after the previous proprietors had gone bankrupt.

Around twenty schools were opened during the late 19th century, although many had only a few pupils or closed within a few years. The largest of the schools were Streete Court School, Wellington House Preparatory School and St Michael's School.

An early experiment with electric lighting took place in December 1878. Six Yablochkoff lamps – a type of arc light – were installed, they were powered by a Garrett engine and were lit for 96 hours. The running costs proved to be uneconomic: they cost £40 9s 5d compared to £16 15s 2d for gas lighting, and there were technical problems.