龋齿（英语：dental caries, tooth decay, cavities, caries，其中caries起源于拉丁文的「腐烂」），俗称蛀牙，指牙齿因细菌活动而造成分解的现象。龋齿时会呈现黄色到黑色之间的不同颜色，其症状包含疼痛与进食困难，并发症包含牙齿周围组织发炎、牙齿丧失与形成牙齿脓肿。

造成龋齿的细菌会经由分解牙齿表面的食物残渣，使牙齿的硬组织（牙釉质、牙本质与牙骨质）分解。食物中的单糖是这些细菌的主要能量来源，因此单糖量含量高的饮食习惯，是龋齿发生的风险因子。如果牙齿矿物质解离速率高过经唾液等来源所引发的牙齿再矿化速率，龋齿就会发生。其他的风险因子包含由糖尿病、干燥综合症与特定药物（抗组织胺药与抗忧郁药物等）引发的唾液分泌量下降。龋齿也与贫穷、疏于清洁口腔与牙周病导致的牙根暴露有关。

Dental caries, also known as tooth decay, cavities, or caries, is a breakdown of teeth due to activities of bacteria. The cavities may be a number of different colors from yellow to black. Symptoms may include pain and difficulty with eating. Complications may include inflammation of the tissue around the tooth, tooth loss, and infection or abscess formation.