核聚变，又称核融合、融合反应或聚变反应，是将两个较轻的核结合而形成一个较重的核和一个很轻的核（或粒子）的一种核反应形式。在此过程中，物质没有守恒，因为有一部分正在聚变的原子核的物质被转化为光子（能量）。核聚变是给活跃的或“主序的”恒星提供能量的过程。

两个较轻的核在融合过程中产生质量亏损而释放出巨大的能量，两个轻核在发生聚变时因它们都带正电荷而彼此排斥，然而两个能量足够高的核迎面相遇，它们就能相当紧密地聚集在一起，以致核力能够克服库仑斥力而发生核反应，这个反应叫做核聚变。

举个例子：两个质量小的原子，比方说氘和氚，在一定条件下（如超高温和高压），会发生原子核互相聚合作用，生成中子和氦-4，并伴随着巨大的能量释放。

In nuclear physics, nuclear fusion is a nuclear reaction in which two or more atomic nuclei come very close and then collide at a very high speed and join to form a new nucleus. The mass of this new nucleus is slightly less than the sum of its constituents, by an amount known as the "binding energy". This is because the compound nucleus is a lower-energy system (a "more favourable" system energetically) than the two parent nuclei. The binding energy is released as photons (energy). This is the energy that is given off by a fusion process. Fusion is the process that powers active or "main sequence" stars, or other high magnitude stars.