Surströmming (pronounced [²sʉːˌʂʈrœmːɪŋ], Swedish for "sour herring") is a type of fermented Baltic Sea herring.
The Baltic herring, known as strömming in Swedish, is smaller than the Atlantic herring, found in the North Sea. Traditionally, the definition of strömming is "herring fished in the brackish waters of the Baltic north of the Kalmar Strait". The herring used for surströmming are caught just prior to spawning.
During a production of surströmming, just enough salt is used to prevent the raw herring from rotting. A fermentation process of at least six months gives the fish a characteristic strong smell and somewhat acidic taste. According to a Japanese study, a newly opened can of surströmming has one of the most putrid food smells in the world, stronger than similarly fermented fish dishes such as the Korean hongeohoe or Japanese kusaya.
At the end of the 1940s, producers lobbied for a royal ordinance (Swedish: förordning) that would prevent incompletely fermented fish from being sold. The decree forbade sales of the current year's production in Sweden prior to the third Thursday in August. While the ordinance is no longer on the books, retailers still maintain the date for the "premiere".