原始森林（英语：Old-growth forest），又称原生林，是指一个森林已经达到非常长久的年龄而没有遭到显着的干扰，从而表现出独特的生态特征，并可能被归类为顶极群落。原生特性包括多样化的树有关的结构，提供多样化的野生动物栖息地，增加了森林生态系统的生物多样性。多样化的树结构的概念包括多层树冠和树冠间隙，很大变化的树的高度和直径，多样的树种和纲，和多种大小的木质残体。

原始森林是陆地生态系统中核心，具有保持生态平衡、涵养水份、调节气候、净化空气等作用。原始森林不仅仅是高大的树木，而是一个综合的生态系统，包动植物间的食物链关系，在原始森林中，某一物种的减少，可以影响其他物种的生存，这种情况在热带雨林中体现更为突出。人们在破坏原始森林以后，即使人工补种了大量的树林，也无法弥补森林被破坏对生态带来的影响的。

An old-growth forest — also termed primary forest, virgin forest, primeval forest, late seral forest, or (in Britain) ancient woodland — is a forest that has attained great age without significant disturbance and thereby exhibits unique ecological features and might be classified as a climax community. Old-growth features include diverse tree-related structures that provide diverse wildlife habitat that increases the bio-diversity of the forested ecosystem. The concept of diverse tree structure includes multi-layered canopies and canopy gaps, greatly varying tree heights and diameters, and diverse tree species and classes and sizes of woody debris.