奶酪（英文: cheese），又名干酪、奶酪，音译称芝士、起司、起士，是多种乳制芝士的通称，有各式各样的味道、口感和形式。干酪以奶类为原料，含有丰富的蛋白质和脂质，乳源包括家牛、水牛、家山羊或绵羊等。制作过程中通常加入凝乳酶，造成其中的酪蛋白凝结，使乳品酸化，再将固体分离、压制为成品。有的干酪在表面或内部布有霉菌，给奶酪带来特别的风味。根据种类不同，奶酪在常温时可为坚硬的固体或柔软的半固体，高温时都会融化，呈粘稠的半液体状态。

奶酪可以分为上百种，乳源（产乳动物所吃的食物也有差异），是否使用巴斯德消毒法，乳脂含量，细菌和霉，处理方法和发酵决定了他们的形状，口感，及味道。奶酪可以用香草，香料，或者烟熏来调味。奶酪中的黄色和红色，例如红莱斯特奶酪，是由于添加了胭脂树的种子。

Cheese is a food derived from milk that is produced in a wide range of flavors, textures, and forms by coagulation of the milk protein casein. It comprises proteins and fat from milk, usually the milk of cows, buffalo, goats, or sheep. During production, the milk is usually acidified, and adding the enzyme rennet causes coagulation. The solids are separated and pressed into final form. Some cheeses have molds on the rind or throughout. Most cheeses melt at cooking temperature.