The Zuiderzee Works (Dutch: Zuiderzeewerken) is a system of dams and dikes, land reclamation and water drainage work, which was the largest hydraulic engineering project undertaken by the Netherlands during the twentieth century. The project involved the damming of the Zuiderzee, a large, shallow inlet of the North Sea, and the reclamation of land in the newly enclosed water using polders. Its main purposes are to improve flood protection and create additional land for agriculture.

The enormous scale of the works required the creation of a joint venture company comprising several large dredging contractors, known as the Maatschappij tot Uitvoering van Zuiderzeewerken. The American Society of Civil Engineers declared the works, together with the Delta Works in the South-West of the Netherlands, as among the Seven Wonders of the Modern World.

The Low Countries have low flat topography, with half the land area below or less than 1 m (3 ft 3 in) above sea level, and have for centuries been subject to periodic flooding by the sea. The seventeenth century saw early proposals to tame and enclose the Zuiderzee, but the ambitious ideas were impractical given the technology then available.

From 1200 to 1900 AD the Dutch reclaimed 940,000 acres (380,000 ha) of land from the sea and 345,000 acres (140,000 ha) by draining lakes, a total of 1,285,000 acres (520,000 ha), but lost 1,400,000 acres (570,000 ha) of land to the Zuiderzee. Hendrik Stevin in 1667 was the first to publish a study ("How the Fury of the North Sea may be stopped and Holland may be protected against it") proposing to drain the Zuiderzee. After the IJ and Haarlemmermeer were drained in the mid-19th century, van Diggelen, Kloppenburg, and Faddegon proposed that the Zuiderzee also be drained. Test drilling by the Zuiderzeevereeniging found that about three quarters of the Zuiderzee would be useful land. Plans were developed during the second half of the nineteenth century to protect areas from the force of the open sea and creating new agricultural land. Cornelis Lely (after whom Lelystad is named) was an ardent supporter, an engineer, and later government minister. A group called the "Zuider Zee Society" began a thorough investigation as to the best means of closing and draining the Zuider Zee in 1886, and in 1891 Lely introduced his plan, which formed the basis for the development of what were to become the Zuiderzee Works. It consisted of a large dam connecting the northern tip of North Holland with the western coast of Friesland and the creation of initially four polders in the northwest, the northeast, southeast (later split in two), and southwest of what would be renamed the IJsselmeer (IJssel-lake). Two major lanes of open water were defined for shipping and drainage. The initial body of water affected by the project was 3,500 square kilometres (1,350 sq mi). Opposition came from fishermen along the Zuiderzee who would lose their livelihood, and from others in coastal areas along the more northerly Wadden Sea. They feared higher water levels as a result of the closure. Other critics doubted whether the project was feasible financially.

Queen Wilhelmina's speech from the throne in 1913, urged reclamation of the Zuiderzee, and the requisite bill was introduced. When Lely became Minister of Transport and Public Works that year, he used his position to promote the Zuiderzee Works and gained support. The government started developing official plans to enclose the Zuiderzee. On 13 and 14 January 1916 the dikes at several places along the Zuiderzee broke under the stress of a winter storm, and the land behind them flooded, as had often happened in previous centuries. This flooding provided the decisive impetus to implement the existing plans to tame the Zuiderzee. In addition, a threatening food shortage during the other stresses of World War I added to widespread support for the project. Lely stressed the importance of extending the area of arable land, and estimated that 500,000 acres (200,000 ha) of good land would be reclaimed from the clay soil, while the regions with a sand bottom would eventually become the IJsselmeer.

On 14 June 1918, the Zuiderzee Act was passed. The goals of the Act were threefold:

Unlike earlier proposals the act intended to preserve part of the Zuiderzee and create large islands, as Lely warned that rerouting the rivers directly to the North Sea might cause inland flooding if storms raised the sea's level. He also wanted to preserve the Zee's fisheries, and for the new land to be accessible by water. The Dienst der Zuiderzeewerken (Zuiderzee Works Department), the government body responsible for overseeing the construction and initial management, was set up in May 1919. In the initial stages of the work heavy clay was dredged out of the open sea and deposited along the track to be followed by the dam. The cost of the undertaking was calculated in 1914, in UK currency, at about £19,000,000 but it soon seemed likely to be very much more. At the time, ten years were allowed for the construction of the main dam and another 20 years for the completion of the four inner enclosures.

The department decided against building the main dam first, proceeding to construct a smaller dam, the Amsteldiepdijk, across the Amsteldiep. This was the first step in rejoining the island of Wieringen to the North Holland mainland. The dike, with a length of 2.5 km (1.6 mi), was built between 1920 and 1924. As with dike building, polder construction was tested on a small scale at the experimental polder at Andijk.