Zwickern or Zwicker is a German fishing card game for two to eight players played in Schleswig-Holstein in North Germany. Its rules first appeared in 1930. It has been described as "a simpler and jollier version of Cassino", which is "exciting and entertaining" and easy to learn. German author, Hans Fallada, who learned it in while in jail at Neumünster, called it "a rather cunning farmer's game from Holstein." The feature that makes it unique among fishing games is its use of up to 6 Jokers.

Zwickern is the primary or only name given in most book sources and rule sets by playing card manufacturers, with only Mensing calling it Zwickeln and Grupp calling it Zwicker, while acknowledging Zwickern and Zwickeln as alternatives. Meanwhile, according to pagat.com, most players on the ground call the game Zwicker and the card packs produced for the game were also labelled Zwicker (see illustration). Another name is Zwick, named after the eponymous feat of sweeping the table, while Zwickel is also known. The names ending in "-n" are verbal nouns i.e. zwickern simply means "playing [the game of] Zwicker" and zwickeln means "playing [the game of] Zwickel." It should not be confused with the Austro-German gambling game of Zwicken.

Zwicker's origins lie in the old English game of Cassino which is first recorded in a 1792 treatise, but spread rapidly to Germany and America. Cassino faded into obscurity in Europe in the late 19th century, but experienced a brief renaissance in America as new variants appeared. Zwicker probably emerged around the turn of the century, but is first recorded in Fritz Lau's 1918 work Elsbe where a character called Hans-Ohm loses 23 groschen playing "Zwickeln". Hans Fallada also recalls that, in 1928, he came across it as "a rather cunning farmer's game from Holstein played with 52 cards and a Joker". The earliest set of rules appeared in 1930 in Robert Hülsemann's Das Buch der Spiele where the game is much as described below in the variant without Jokers and looks very much like an elaboration of Royal Cassino. Originally, it was played with just a standard 52-card pack but now it is commonly played with 3, 4 or even 6 jokers. Bespoke 58-card packs have been in production since at least the 1950s, but they now appear to be discontinued, the last manufacturer, NSV, no longer offering them in its range.

In 1935, the 52-card game (presumably without Jokers) was described as popular in Schleswig-Holstein, especially in the Holsteinish regions of Krempermarsch, Wilstermarsch and Dithmarschen as well as the Schleswigian regions of North Frisia and the Eiderstedt peninsula. It was almost never played for money, but usually for a glass of beer or a Grog. Dithmarschen may be where the game originated, but it must have spread and become widely popular in the region in order for special packs to be made for it.

In recent times, it has been played in North Frisia – for example, in Neukirchen, Leck and on the island of Sylt – as well as further east in the county of Schleswig-Flensburg at Sieverstedt Sillerup, Großenwiehe and Schafflund. It is still recorded in Holstein at Tappendorf and Windbergen, near its place of origin in Dithmarschen.

The game is very similar to Swedish Byggkasino.

Jokers were not originally used in this game, but it is now one of few games that is played with up to six in a single pack. From the 1950s, special Zwicker packs were manufactured for this purpose, initially by ASS Altenburger and, later, also by NSV. The latter firm made them until around 2020. Today they are no longer available and two packs of standard cards each of 3 Jokers with the same back designs are needed to play the versions with 4 to 6 Jokers. Where Jokers used, they play a major role in the game and are usually the most valuable counting cards. Much of the literature follows Hülsemann in allowing no Jokers; in Grupp and Parlett they are optional; but in practice, all accounts by real players include the use of Jokers, albeit their values vary.

According to Grupp (1975), the normal rule is that, whether on the table or played from the hand, they are wild and represent any card chosen by the player. Optional rules to limit these powers include: if a Joker is the only card on the table, it may only be taken by another Joker; if a Joker is used in sweeping all cards from the table, it does not count as a zwick; or even that Jokers may never be picked up from the table, which effectively prevents any more zwicks being made. Parlett follows Grupp, but McLeod points out that no players in Schleswig-Holstein treat the Jokers as wild and believes that Grupp invented the wild Joker rule to explain the Jokers in the pack.