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The Afsluitdijk (English: Closure Dike, Frisian: Ofslútdyk) is a major causeway in the Netherlandsmarker, constructed between 1927 and 1933 and running from Den Oever on Wieringenmarker in North Holland province, to the village of Zurichmarker (mun. Wûnseradielmarker) in Friesland province, over a length of 32 km (20 miles) and a width of 90 m, at an initial height of 7.25 m above sea-level.

It is a fundamental part of the larger Zuiderzee Works, damming off the Zuiderzeemarker, a salt water inlet of the North Seamarker and turning it into the fresh water lake of the IJsselmeermarker.

Previous experiences had demonstrated that till (boulder clay), rather than just sand or clay, was the best primary material for a structure like the Afsluitdijk, with the added benefit that till was in plentiful supply in the area; it could be retrieved in large quantities by simply dredging it from the bottom of the Zuiderzee. Work started at four points: on both sides of the mainland and on two specially made construction-islands (Kornwerderzandmarker and Breezand) along the line of the future dike.

From these points, the dike slowly grew by ships depositing till into the open sea in two parallel lines. Sand was then poured in between the two dikes and as it emerged above the surface was then covered by another layer of till. The nascent dike was then strengthened from land by basalt rocks and mats of willow switch at its base. The dike could then be finished off by raising it further with sand and finally clay for the surface of the dike, on which grass was planted.

Construction progressed better than expected; at three points along the line of the dike there were deeper underwater trenches where the tidal current was much stronger than elsewhere. These had been considered to be major obstacles to completing the dike, but all of them proved to be relatively straightforward. On May 28, 1932, two years earlier than initially thought, the Zuiderzee ceased to be, as the last tidal trench of the Vlieter was closed by a final bucket of till. The IJsselmeer was born, even though it was still salty at the time.

The dike itself however was not finished yet as it still needed to be brought up to its required height and a road linking Friesland and North Holland (the current A7/E22 motorway) also remained to be built. It would not be until September 25, 1933, that the Afsluitdijk was officially opened, with a monument designed by architect Dudok marking the spot where the dike had been closed. The amount of material used is estimated at 23 million m³ of sand and 13.5 million m³ of till and over the years an average of around four to five thousand workers were involved with the construction every day, relieving some of the unemployment following the Great Depression.
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Beside the dike itself there was also the necessary construction of two complexes of shipping locks and discharge sluices at both ends of the dike. The complex at Den Oever includes the Stevin lock (named after Hendric Stevin, a son of mathematician and engineer Simon Stevin) and three series of five sluices for discharging the IJsselmeer into the Wadden Sea; the other complex at Kornwerderzand is composed of the Lorentz locks (named after Hendrik Lorentz, the famous physicist, who personally did the calculations of the tides that were crucial to the construction of the Afsluitdijk) and two series of five sluices, making a total of 25 discharge sluices. Periodically discharging the lake is necessary since it is continually fed by rivers and streams (most notably the IJssel river that gives its name to the lake) and polders draining their water into the IJsselmeer.

During the period 12–13 May 1940, it was the site of the Battle of the Afsluitdijkmarker.

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