A belt buckle is a buckle, a clasp for fastening two ends, such as of straps or a belt, in which a device attached to one of the ends is fitted or coupled to the other. The word enters Middle English via Old French and the Latin buccula or "cheek-strap," as for a helmet. Belt buckles and other fixtures are used on a variety of belts, including cingula, baltea, baldrics and later waist-belts.

Belt buckles go back at least to the Iron Age and a gold "great buckle" was among the items interred at Sutton Hoo. Primarily decorative "shield on tongue" buckles were common Anglo-Saxon grave goods at this time, elaborately decorated on the "shield" portion and associated only with men. One such buckle, found in a 7th-century grave at Finglesham, Kent during excavations by Sonia Chadwick Hawkes in 1964 bears the image of a naked warrior standing between two spears wearing only a horned helmet and belt.

Frame-style buckles are the oldest design. In a frame-and-prong buckle the prong attaches to one end of the frame and extends "away" from the wearer through a hole in the belt, where it anchors against the opposite side of the frame. The oldest styles have a simple loop or D-shaped frame (see: D-ring), but "double-loop" or "center post" buckles whose prongs attach to a fixed center section appear in the 8th century. Very small buckles with removable center pins and chapes were introduced and used on shoes, beginning in the 17th century, but not often for waist-belts. A "chape" is the fixed cover or plate which attaches buckle to belt while the "mordant" or "bite" is the adjustable portion.

Plate-style buckles are common on western military belts of the mid-19th century, which often feature a three-hook clasp: two hooks fitting into one end of the belt and a third into the other. Officers might have a similar but more intricate clasp-style closure that featured two interlocking metal parts. In practice, the term "belt plate" refers to any flat, decorated surface on such a clasp. These precede development of modern "western-style" buckles, which feature a hinged frame affixed to one end of the belt and a simple hook clasp which enters the belt hole toward the wearer but leaves most of the buckle on the "outside" of the belt, providing an ample surface for decoration. The distance between the fixed frame or chape of a plate buckle and its adjustment prong is called the "throw."

Box-frame buckles are a 20th-century style of military friction buckle, common on webbed belts. The box-frame buckle consists of three parts (front, back and post). An adjustable captive post sits perpendicular to the belt to press it against the outer "box," which completely surround the webbing and minimize accidental adjustments should part of the belt snag on something. There may or may not be a metal tip on the opposite "tongue" end of the belt for easier insertion.

O-Ring/D-Ring buckles use one or two rings to form the buckle. The belt is fastened by threading through the ring(s). This is used with braided, webbing, and canvas belts.

Quick-release buckles are found in various forms and are typically used on webbing equipment.

Snap/Side release buckles use male and female ends to snap together. They are more functional and often used for outdoor activities.