嗜酸乳杆菌（学名：Lactobacillus acidophilus），又称A菌，是乳杆菌属的一种。A菌适合生存在酸性、30°C（86°F）的环境中，存在人类、动物的肠胃道和口腔，和大部分的乳酸菌一样能够将乳糖转变为乳酸。其相关的菌种还能产生乙醇、二氧化碳、乙酸。A菌无法在温度与湿度过高、阳光直射的环境生存。能在相对较低pH值（pH 5.0以下）稳定生长，最佳生长温度约为37°C。部分的嗜酸乳杆菌菌种带有益生菌的特质。这些菌种在商业上因此用于多种奶制品中，有时还会跟嗜热链球菌（Streptococcus thermophilus）和保加利亚亚种德氏乳杆菌（Lactobacillus delbrueckii subsp. bulgaricus）配搭用于制作嗜酸型的酸奶饮品。

Lactobacillus acidophilus (New Latin 'acid-loving milk-bacillus') is a species of gram positive bacteria in the genus Lactobacillus. L. acidophilus is a homofermentative, microaerophilic species, fermenting sugars into lactic acid, and grows readily at rather low pH values (below pH 5.0) and has an optimum growth temperature of around 37 °C (99 °F). L. acidophilus occurs naturally in the human and animal gastrointestinal tract and mouth. Some strains of L. acidophilus may be considered to have probiotic characteristics. These strains are commercially used in many dairy products, sometimes together with Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus in the production of acidophilus-type yogurt. Its genome has been sequenced.