Abstract: An increasing number of studies have demonstrated that lactic acid, a key signaling molecule, plays a critical role in tumorigenesis, immunotherapy response, and cellular metabolism regulation. Immunotherapy has become a mainstay of cancer treatment; however, most patients fail to respond, which may stem from the tumor immune microenvironment (TIME). Lactic acid, a central metabolic modulator of the tumor immune response, accumulates in the TIME and suppresses immune cell activity, thereby promoting tumor growth and immune escape. Targeting lactate metabolism and its associated pathways presents a novel strategy to overcome immunotherapy resistance. This article reviews current research on the regulatory mechanism of lactic acid in the TIME and immunotherapy, providing new ideas for combating tumor immunotherapy resistance.