The line of battle is a tactic in naval warfare in which a fleet of ships forms a line end to end. The first example of its use as a tactic is disputed—it has been variously claimed for dates ranging from 1502 to 1652. Line-of-battle tactics were in widespread use by 1675.

Compared with prior naval tactics, in which two opposing ships closed on one another for individual combat, the line of battle has the advantage that each ship in the line can fire its broadside without fear of hitting a friendly ship. This means that in a given period, the fleet can fire more shots. Another advantage is that a relative movement of the line in relation to some part of the enemy fleet allows for a systematic concentration of fire on that part. The other fleet can avoid this by manoeuvring in a line itself, with a result typical for sea battles since 1675: two fleets sail alongside one another (or on the opposite tack).