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 away from the vertical, the engine is sometimes called a slant-six (although this is also a Chrysler-specific design). 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.