A Roman dodecahedron or Gallo-Roman dodecahedron is a small hollow object made of copper alloy which has been cast into a regular dodecahedral shape with twelve flat pentagonal faces. Each face has a circular hole of varying diameter in the middle, the holes connecting to the hollow center, and each corner has a protruding knob. Roman dodecahedra date from the 2nd to 4th centuries AD and their purpose remains unknown. They rarely show signs of wear, and do not have any inscribed numbers or letters.

Roman dodecahedrons are cast in metal, usually a copper alloy, although one silver example is known. They are in the shape of regular dodecahedrons, with the addition of a small metal ball at each vertex; each face has a circular hole in the centre. The dodecahedrons vary in size from 4 to 11 cm; the holes in their faces have diameters ranging from 6 mm to 40 mm. The lightest known example weighs 35 grams; the heaviest 1000 grams. Most are decorated with circles, lines, and dots, but no letters or numbers are known to be marked on any of the objects. One dodecahedron was found in a tomb alongside a bone object which might have been a kind of handle; however it was in too bad a condition to excavate and preserve.

The first dodecahedron was found in 1739. Since then, at least 130 similar objects have been found in Austria, Belgium, France, Germany, Hungary, Luxembourg, the Netherlands, Switzerland and the United Kingdom, but not in the Roman heartland in Italy. In 2023, a dodecahedron in excellent condition was found by amateur archeologists in the small village of Norton Disney in Lincolnshire, UK, bringing the total to 33 of such objects found on the territory of Roman Britain. It is on exhibition at the Lincoln Museum, Lincolnshire until October 2024.

The purpose of Roman dodecahedrons has been much debated: more than fifty possible explanations have been published. Identifying their purpose with certainty is made more difficult as no mention of dodecahedrons has been found in contemporary accounts or pictures of the time. Speculative uses include as a survey instrument for estimating distances to (or sizes of) distant objects, though this is questioned as there are no markings to indicate that they would be a mathematical instrument; as spool knitting devices for making gloves (though the earliest known reference to spool knitting is from 1535, and this would neither explain the use of bronze, nor the apparently similar icosahedron which is missing the holes necessary for spool knitting); as part of a child's toy.

Several dodecahedra were found in coin hoards, suggesting either that their owners considered them valuable objects, or that their use was connected with coins — as, for example, for easily checking coins fit a certain diameter and were not clipped. It has been suggested that they might have been religious artifacts, or even fortune-telling devices. This latter speculation is based on the fact that most of the examples have been found in Gallo-Roman sites. It has also been suggested that they might have been an object to test the skill of a metalsmith, perhaps as part of a portfolio to demonstrate their capabilities to customers or as a way to qualify for a certain status in a collegium (guild). This speculation is based on the historic cost of bronze and the level of skill necessary to cast such an object. Some 19th-century antiquarians speculated that they might be weapons, such as the head of a mace or a metal bullet, but other scholars have suggested that the dodecahedra are too light to make an effective weapon.

A Roman icosahedron has also been discovered after having long been misclassified as a dodecahedron. This icosahedron was excavated near Arloff in Germany and is currently on display in the Rheinisches Landesmuseum in Bonn. In 2009, metal-detectorists in France reported the discovery of an object at Saint-Trivier-de-Courtes, similar to the Roman dodecahedrons but spherical with triangular and pentagonal holes.

Smaller dodecahedra with the same features (holes and knobs) and made from gold have been found in South-East Asia along the Maritime Silk Road and the earliest items appear to be from the Roman epoch. Examples include those uncovered in Óc Eo, Vietnam, by Louis Malleret, who concluded that the objects represented the influence of Mediterranean trade on the Funan economy. Similar decorative gold dodecahedrons have been found in the Pyu city-states and Khao Sam Kaeo.

Media related to Roman dodecahedron at Wikimedia Commons

• "History Mystery: Ancient Dodecahedron's Purpose Remains Secret" by Alexandria Hein, Fox News, June 10, 2011