史瓦西半径（Schwarzschild radius）是任何具有质量的物质都存在的一个临界半径特征值。在物理学和天文学中，尤其在万有引力理论、广义相对论中，它是一个非常重要的概念。1916年卡尔·史瓦西首次发现了史瓦西半径的存在，这个半径是一个球状对称、不自转又不带电荷的物体的重力场的精确解。该值的含义是，如果特定质量的物质被压缩到该半径值之内，将没有任何已知类型的力（如简并压力）可以阻止该物质自身的重力将自己压缩成一个奇点。

对符合条件（即不自转、不带电）的任何物体的史瓦西半径皆与其质量成正比。理论上，太阳的史瓦西半径约为3千米，地球的史瓦西半径只有约9毫米。

The Schwarzschild radius (sometimes historically referred to as the gravitational radius) is the radius of a sphere such that, if all the mass of an object were to be compressed within that sphere, the escape velocity from the surface of the sphere would equal the speed of light. An example of an object where the mass is within its Schwarzschild radius is a black hole. Once a stellar remnant collapses to or below this radius, light cannot escape and the object is no longer directly visible, thereby forming a black hole. It is a characteristic radius associated with every quantity of mass. The Schwarzschild radius was named after the German astronomer Karl Schwarzschild, who calculated this exact solution for the theory of general relativity in 1916.