Tzatziki, tzadziki, or tsatsiki (τζατζίκι) is a Greek meze or appetizer, possibly of Turkish origin, also used as a sauce for souvlaki and gyros. Tzatziki is made of strained yoghurt — in Greece and Turkey usually sheep's-milk or goat's-milk yoghurt — to which are added cucumbers (either pureed and strained or seeded and finely diced), garlic, salt and usually olive oil, sometimes vinegar, and dill, parsley or mint. Olive oil, olives, and herbs are often used as garnishes as well.

In Cyprus, the dish is known colloquially as ttalattouri (cf. tarator), and recipes often include less garlic and includes the herb mint unlike the Greek counterpart. Tzatziki is always served cold.

Tzatziki is often served with bread (loaf or pita) as part of the first course of a meal. Tzatziki is also used as a sauce for souvlaki and gyros, in which case it may be called cucumber sauce (especially in the U.S.).

The Greek word is derived from the Turkish cacık, which means a form of chutney (cacık, the Turkish side dish with similar ingredients, is diluted).

In Bulgaria and the Republic of Macedonia, the same dish is known as "dry tarator" (сух таратор) or "Snezhanka" salad (салата "Снежанка"), which means "snow white salad", and is served as an appetizer. During preparation, the yoghurt (кисело мляко) is hung for several hours in a kerchief and loses about half of its water (drained yoghurt, цедено кисело мляко). The cucumbers, garlic, minced walnuts, salt and vegetable oil are then added.

Similar dishes in Iraq are known as jajeek, normally served as meze alongside alcoholic drinks, especially Arak, an Ouzo-like drink made out of dates.

A variation in the Caucasus mountains, called ovdukh, uses kefir instead of the yogurt, thus creating a refreshing summer drink. This can be poured over a mixture of vegetables, eggs and ham to create a variation of okroshka, sometimes referred to as a 'Caucasus okroshka'.

A similar dish is made in Iran called mast-o-khiar, literally meaning yogurt with cucumber. It is made using a thicker yogurt, which is mixed with sliced cucumber, garlic, and mint (sometimes chopped nuts are also added).

In America, tzatziki is sometimes made with sour cream instead of yogurt. However, Greek-style strained yogurt, of various fat levels, is now sold in many supermarkets, eliminating the most time-consuming step of preparing tzatziki in the traditional way and allowing the cook to quickly prepare a yogurt-based tzatziki with a thick consistency.

Cacık may also be compared with raita and pachadi in India, all are served as a refreshing appetizer along with other dishes.

The Sephardic Jewish name for this sauce, at least in Greece, is tarator.