Lactobacillus casei is a species of genus Lactobacillus found in the human intestine and mouth. As a lactic acid producer, it has been found to assist in the propagation of desirable bacteria. This particular species of lactobacillus is documented to have a wide pH and temperature range, and complements the growth of L. acidophilus, a producer of the enzyme amylase (a carbohydrate-digesting enzyme). It is known[by whom?] to improve digestion and reduce lactose intolerance and constipation.

The most common application of L. casei is industrial, specifically for dairy production. However, a team of scientists from Simón Bolívar University in Caracas, Venezuela found that, by using Lactobacillus casei bacteria in the natural fermentation of beans, the beans contained lower amounts of the compounds causing flatulence upon digestion.

Lactobacillus casei is typically the dominant species of non-starter lactic acid bacteria (NSLAB) present in ripening Cheddar cheese, and, recently, the complete genome sequence of L. casei ATCC 334 has become available. L. casei is also the dominant species in naturally fermented Sicilian green olives.