Strained yogurt, Greek yogurt,[2] yogurt cheese, or sack yoghurt is yogurt that has been strained to remove most of its whey, resulting in a thicker consistency than unstrained yogurt, while preserving yogurt's distinctive sour taste. Like many types of yogurt, strained yogurt is often made from milk that has been enriched by boiling off some of its water content, or by adding extra butterfat and powdered milk. In Europe and North America, it is often made from low-fat or fat-free milk. In Iceland, a similar product named skyr is made.

Strained yogurt is generally marketed in North America as "Greek yogurt" and in Britain as "Greek-style yoghurt",[3] though strained yogurt is also widely eaten in Levantine, Eastern Mediterranean, Middle Eastern, Central Asian and South Asian cuisines, wherein it is often used in cooking (as it is high enough in fat content to avoid curdling at higher temperatures). Such dishes may be cooked or raw, savoury or sweet. Due to the straining process to remove excess whey, even non-fat varieties of strained yogurt are much thicker, richer, and creamier than yogurts that have not been strained. Since the straining process removes the whey, or fluid, from the milk solids, it requires substantially more plain yogurt to produce a cup of strained yogurt, so the cost to make it is increased accordingly. Thickeners, such as pectin, locust bean gum, starches or guar gum, listed in the ingredients indicate straining was not the method used to consolidate the milk solids. Marketing of Greek or Greek-style yogurt in the United States allows the use of more thickeners instead of straining, so there is little difference between the yogurt of years prior to the introduction of strained yogurt other than an increase of thickeners, even though the price is now higher for the original, unstrained products. In western Europe and the US, strained yogurt has increased in popularity compared to unstrained yogurt. Since the straining process removes some of the lactose, strained yogurt is lower in sugar than unstrained yogurt.[4]

It was reported in 2012 that most of the growth in the $4.1 billion US yogurt industry came from the strained yogurt sub-segment, typically marketed as "Greek yogurt".[5][6] In the US there is no legal definition of Greek yogurt, and yogurt thickened with thickening agents may also be sold as "Greek yogurt" even though it is not necessarily strained yogurt.[7]