Lactobacillus bulgaricus (LBB) is one of several bacteria used for the production of yoghurt (yogurt). It is named for Bulgaria, the country where it was first used (it thrives freely on the Balkan Peninsula). The bacterium feeds on milk and produces lactic acid which also helps to preserve the milk. It breaks down lactose and is often helpful to sufferers of lactose intolerance, whose digestive systems lack the enzymes to break down lactose to simpler sugars. While fermenting milk, Lactobacillus bulgaricus produces acetaldehyde which perfumes yogurt. One of the biggest importers of the bacterium is Japan.

According to legend, a Bulgarian shepherd in the Balkan Mountains (Stara Planina) who did not have enough vessels to hold all the milk fom his sheep made a bag from a fresh lamb's hide and put the extra milk there. The next morning he found yoghurt in the bag, most likely generated by Lactobacillus Bulgaricus. Other legends suggest that yoghurt was known even to the Bulgars (a nomadic tribe, also known as the proto-Bulgarians), along with kumis, a refreshing, slightly alcoholic drink from mare's milk.

Between 13th and 18th century AD Bulgaria was under the rule of the Ottoman Empire, and Turkish was the official language. The word yoghurt most probably came into English from the Turkish "yogurut".

In most Western countries, commercial yogurt production employs various starches, gums, gels, and stabilizers and in some instances may include little or no live bacteria culture. This kind of yogurt is often considered inferior to the fully-cultured variety.

The bacterium was first identified in 1905 by the Bulgarian doctor Stamen Grigorov .