A Roman mosaic is a mosaic made during the Roman period, throughout the Roman Republic and later Empire. Mosaics were used in a variety of private and public buildings, on both floors and walls, though they competed with cheaper frescos for the latter. They were highly influenced by earlier and contemporary Hellenistic Greek mosaics, and often included famous figures from history and mythology, such as Alexander the Great in the Alexander Mosaic.

A large proportion of the surviving examples of wall mosaics come from Italian sites such as Pompeii and Herculaneum. Otherwise, floor mosaics are far more likely to have survived, with many coming from the fringes of the Roman Empire. The Bardo National Museum in Tunis has an especially large collection from large villas in modern Tunisia.

Perhaps the earliest examples of Greco-Roman mosaic floors date to the late Republican period (2nd century BC) and are from Delos, Greece. Witts claims that tessellated pavements, using tesserae, were used in Europe from the late fifth to early fourth centuries BC. This is contradicted by Ruth Westgate, who contends that the earliest tessellated mosaics of the Hellenistic period date to the 3rd century BC, with the 2nd to early 1st-century BC mosaics of Delos constituting roughly half of the known examples. Hetty Joyce and Katherine M. D. Dunbabin concur with this assessment, asserting that the transition from pebble mosaics to more complex tessellated mosaics originated in Hellenistic-Greek Sicily during the 3rd century BC, developed at sites such as Morgantina and Syracuse. The earliest known pebble mosaics and use of chip pavement are found at Olynthus in Greece's Chalcidice, dated to the 5th to 4th centuries BC, while other examples can be found at Pella, capital of Macedon, dated to the 4th century BC.

The earliest mosaics of Roman Pompeii, dated to the Pompeian First Style of wall painting in the late 2nd and early 1st centuries BC, were clearly derived from the Hellenistic Greek model. However, they contained far more figured scenes on average, less abstract design, the absence of lead strips, as well as an almost complete lack of complex, three-dimensional scenes utilizing polychromy until the Pompeian Second Style of wall painting (80–20 BC).

The mosaics in the Villa Romana del Casale (c. 300 AD) from Roman Sicily perhaps represent the hallmark of mosaic art in the Late Imperial period. The mosaic decoration of the local palace complex culminates in the gallery, which contains a scene of animal hunting and fighting covering an area of 3,200 square feet (300 m²).

Roman mosaics are constructed from geometrical blocks called tesserae, placed together to create the shapes of figures, motifs and patterns. Materials for tesserae were obtained from local sources of natural stone, with the additions of cut brick, tile and pottery creating coloured shades of, predominantly, blue, black, red, white and yellow. Polychrome patterns were most common, but monochrome examples are known. Marble and glass were occasionally used as tesserae, as were small pebbles, and precious metals like gold.

Mosaic decoration was not just confined to floors but featured on walls and vaults as well. Traces of guidelines have been found beneath some mosaics, either scored into or painted onto the mortar bedding. The design might also be pegged out in string, or mounted in a wooden frame.

The collapse of buildings in antiquity can, paradoxically, both irrevocably destroy mosaics or protect and preserve them.