Isinglass is a substance obtained from the swim bladders of fish (especially sturgeon) that is used for the clarification of wine and beer.

Isinglass finings are used extensively as a processing aid in the British brewing industry to accelerate the fining (clarification) of beer. They are used particularly in the production of cask conditioned beers (real ale) although there are a few real ales available which are not fined using isinglass. The finings, a pure form of gelatine, flocculate (coagulate into a spongy mass) with the live yeast which are suspended in the beer and settle to the bottom of the cask where the floc remains. Left to itself, beer will clear naturally, however the use of isinglass finings accelerates the process.

Non-cask beers (destined for kegs, cans or bottles) are often pasteurized and filtered. The yeast in these beers tends to settle to the base of the storage tank naturally so the sediment from these beers can often be filtered without using isinglass. Some breweries do still use isinglass finings for non-cask beers however, especially when attempting to repair bad batches.

Although very little isinglass remains in the beer which is drunk, many vegetarians consider beers which are processed with these finings to be unsuitable for vegetarian diets. A list of vegetarian beers identifies many beers in which isinglass finings are not used and which are therefore suitable for vegetarians.

A vegetarian alternative to Isinglass is Irish moss, which is derived from seaweed. However, the two agents are used in different ways. Isinglass is added at end of the brewing process (before bottling), whereas Irish Moss added to the hot wort while it is being boiled, and primarily reduces hazes caused by proteins. As the two agents act in different ways, some beers will make use of both fining agents.