林木线（tree line或timberline），简称为林线，亦有人称之为「森林界线（Forest Line/limit）」。该线为生态学、环境学及地理学的一个概念，乃指分隔植物因气候、环境等因素而能否生长的界线。在该线以内，植物可如常生长；然而一旦逾越该线，大部份植物均会因风力、水源、土壤及／或其他气候原因而无法生长。

按林木线出现的原因及地点，林木线又可细分为阿尔卑斯林木线（即高山林木线）、沙漠林木线、高山沙漠林木线、北极林木线及南极林木线等多类。林木线的界线常因气候的改变而有所变更。

The tree line is the edge of the habitat at which trees are capable of growing. It is found at high elevations and in frigid environments. Beyond the tree line, trees cannot tolerate the environmental conditions (usually cold temperatures or lack of moisture). The tree line should not be confused with a lower timberline or forest line, which is the line where trees form a forest with a closed canopy.