Semi-automatic (sometimes written as one word, without the hyphen) firearms are those which can fire successive shots without requiring the shooter to manually chamber new rounds or recock the hammer. "Self-loading" and "auto-loading" are sometimes used as equivalent terms. This enables shooters to fire more quickly, getting off more shots in a given time period, but the additional mechanisms required to give guns this capability may make them more bulky, expensive, difficult to maintain, or prone to failure.

When a round is fired from a semiautomatic firearm, some of the force of the shot is harnessed to eject the cartridge in the chamber, load the next round from the magazine, and return the hammer to a firing position. Thus, the gun is ready to be fired again without the need for any action on behalf of the shooter. Semi-automatic operation differs from full automatic in that the trigger must still be pulled once for each round fired; pulling the trigger of a fully automatic weapon will cause it to fire repeatedly until the trigger is released or ammunition is depleted.

If asked to visualize a pistol, most people will picture a semiautomatic: fully automatic firearms employing pistol ammunition are typically thought of as submachine guns, and single-shot or manual-action handguns are relatively rare in modern times. By contrast, there are a wide variety of semiautomatic pistols on the market, produced by several manufacturers and chambered for all common and some obscure handgun calibers. Most of these guns are, if not direct descendants, heavily influenced by the seminal Colt 1911 or Browning HP designs. Revolvers, with few exceptions, are not true semiautomatics: while after each shot another chamber is rotated into alignment with the barrel, the hammer is not recocked, and must be cocked again, either as part of the trigger action (in double action revolvers), or manually (in single action models).

Most modern automatic military rifles have a selectable semi-automatic mode, and "civilian" versions of such rifles are usually exclusively semiautomatic. True machine guns are by definition automatic; while it may be possible to convert them to semiautomatic operation, as with civilian versions of the .50 cal. Browning HMG, this rather defeats the point. Semiautomatic shotguns, while less iconic, do exist (the semi/pump hybrid SPAS-12 probably being the most famous example), and are preferred for target shooting and human-against-human conflict.

Some semiautomatic firearms can, with skill and aftermarket parts, be modified to fire in full automatic and/or burst fire modes. This may be illegal in some jurisdictions, however, and shooters will often find that any increase in effective rate of fire is more than offset by decreased accuracy, especially if they have not been trained in these modes of fire.