A battering ram is a siege engine that originated in ancient times and was designed to break open the masonry walls of fortifications or splinter their wooden gates. In its simplest form, a battering ram is just a large, heavy log carried by several people and propelled with force against an obstacle; the ram would be sufficient to damage the target if the log were massive enough and/or it were moved quickly enough (that is, if it had enough momentum). Later rams encased the log in an arrow-proof, fire-resistant canopy mounted on wheels. Inside the canopy, the log was swung from suspensory chains or ropes.

Rams proved effective weapons of war because at the time wall-building materials such as stone and brick were weak in tension, and therefore prone to cracking when impacted with force. With repeated blows, the cracks would grow steadily until a hole was created. Eventually, a breach would appear in the fabric of the wall, enabling armed attackers to force their way through the gap and engage the inhabitants or garrison of the fortified place.

The introduction in the later Middle Ages of siege cannons, which harnessed the explosive power of gunpowder to propel weighty stone or iron balls against fortified obstacles, spelled the end of battering rams and other traditional siege weapons. Smaller, hand-held versions of battering rams are still used today by law enforcement officers and military personnel to break open locked doors.

A capped ram is a battering ram that has an accessory at the head (usually made of iron or steel and sometimes punningly shaped into the head and horns of an ovine ram) to do more damage to a building. It was much more effective at destroying enemy walls and buildings than an uncapped ram but was heavier to carry.

The earliest depiction of a possible battering ram is from the tomb of the 11th Dynasty Egyptian noble Khety, where a pair of soldiers advance towards a fortress under the protection of a mobile roofed structure, carrying a long pole that may represent a simple battering ram.

During the Iron Age, in the ancient Middle East and Mediterranean, the battering ram's log was slung from a wheeled frame by ropes or chains so that it could be made more massive and be more easily bashed against its target. Frequently, the ram's point would be reinforced with a metal head or cap while vulnerable parts of the shaft were bound with strengthening metal bands. Vitruvius details in his text De architectura that Ceras the Carthaginian was the first to make a ram with a wooden base with wheels and a wooden superstructure, with the ram hung within. This structure moved so slowly, however, that he called it the testudo (Latin for "tortoise").

Another type of ram was one that maintained the normal shape and structure, but the support beams were instead made of saplings that were lashed together. The frame was then covered in hides as normal to defend from fire. The only solid beam present was the ram that was hung from the frame. The frame itself was so light that it could be carried on the shoulders of the men transporting the ram, and the same men could beat the ram against the wall when they reached it.

Many battering rams had curved or slanted wooden roofs and side-screens, covered in protective materials, usually fresh wet hides. These canopies reduced the risk of the ram being set on fire, and protected the operators of the ram from arrow and javelin volleys launched from above.