直列六缸发动机（英语：Straight-six engine），简称直六、I6或L6，是指拥有6个气缸，且一字排开的引擎结构。世爵汽车于1903年成为第一间生产直列六缸引擎的车厂，至1909年全世界已经有80间车厂使用这设计。直列六缸引擎只有单一曲轴，设计上出力较为平衡，但引擎较长，在V6发动机普及后已经被大多数的车厂放弃使用。BMW和沃尔沃汽车是小数仍然生产直列六缸发动机的车厂，前者配置于其高性能的前置后驱房车，后者配置于入门级的前置前驱房车。不过，中重型商用车辆的柴油引擎则以直列六缸为主流，因其引擎摆放方式跟乘用车有分别。

The straight-six engine or inline-six engine (often abbreviated I6 or L6) is an internal combustion engine with the cylinders mounted in a straight line along the crankcase with all the pistons driving a common crankshaft (straight engine).

The bank of cylinders may be oriented at any angle, and where the bank is inclined to the vertical, the engine is sometimes called a slant-six. The straight-six layout is the simplest engine layout that possesses both primary and secondary mechanical engine balance, resulting in much less vibration than engines with fewer cylinders.