The Dionysia (/ˌdaɪ.əˈnɪzi.ə, ˌdaɪ.əˈnɪʃi.ə, ˌdaɪ.əˈnɪʃə/; Greek: Διονύσια) was a large festival in ancient Athens in honor of the god Dionysus, the central events of which were processions and sacrifices in honor of Dionysus, the theatrical performances of dramatic tragedies and, from 487 BC, comedies. It was the second-most important festival after the Panathenaia. The Dionysia actually consisted of two related festivals, the Rural Dionysia and the City Dionysia, which took place in different parts of the year.

The Dionysia was originally a rural festival in Eleutherae, Attica (Διονύσια τὰ κατ' ἀγρούς – Dionysia ta kat' agrous), celebrating the cultivation of vines. Archaeological evidence suggests that theatres for the Rural Dionysia had been constructed as early as the 6th century BCE, but the festival is generally believed to have been celebrated even before that. This "rural Dionysia" was held during the winter, in the month of Poseideon (the month straddling the winter solstice, i.e., Dec.–Jan.), although it is also suggested that festivals took place in the Spring time as well. The central event was the pompe (πομπή), the procession, in which phalloi (φαλλοί) were carried by phallophoroi (φαλλοφόροι). Also participating in the pompe were kanephoroi (κανηφόροι – young girls carrying baskets), obeliaphoroi (ὀβελιαφόροι – who carried long loaves of bread), skaphephoroi (σκαφηφόροι – who carried other offerings), hydriaphoroi (ὑδριαφόροι – who carried jars of water), and askophoroi (ἀσκοφόροι – who carried goatskin bags of wine).

After the pompe procession was completed, there were contests of dancing and singing, and choruses (led by a choregos) would perform dithyrambs. Some festivals may have included dramatic performances, possibly of the tragedies and comedies that had been produced at the City Dionysia the previous year. This was more common in the larger towns, such as Piraeus, Eleusis and Icaria/Ikarion.

The festival was celebrated in urban towns outside of the rural setting such as in Kollytos and Peiraieus, indicating that it became less of a celebration of rural communities and more so a celebration of agrarian culture as a whole. Because the various towns in Attica held their festivals on different days, it was possible for spectators to visit more than one festival per season. It was also an opportunity for Athenian citizens to travel outside the city if they did not have the opportunity to do so during the rest of the year. This also allowed travelling companies of actors to perform in more than one town during the period of the festival.

The comic playwright Aristophanes parodied the Rural Dionysia in his play The Acharnians by making a mockery of the pompe and the significance of phalluses. His description is considered the earliest surviving documentation of the festival in Athens and has been used as a reference on its proceedings. Plutarch in his treatise De cupiditate divitiarum, commented on the simple nature of the celebration of Rural Dionysia in antiquity. Aeschines makes reference to the performance of comedies during the Rural Dionysia in Kollytos in his speech Against Timarchus. The festival has also been mentioned in writing by Theophrastus in Adoleschia, Plutarch additionally in Moralia, and in Plato's Republic.

The City Dionysia (Dionysia ta en Astei – Διονύσια τὰ ἐν Ἄστει, also known as the Great Dionysia, Dionysia ta Megala – Διονύσια τὰ Μεγάλα) was the urban part of the festival. It was established during the tyranny of Peisistratus in the 6th century BC due to his recognition of the Cult of Dionysius as a national cult, the promotion of performative arts, and the reformation of the festival. This festival was held probably from the 10th to the 16th of the month Elaphebolion (the lunar month straddling the vernal equinox, i.e., Mar.-Apr in the solar calendar), three months after the rural Dionysia, probably to celebrate the end of winter and the harvesting of the year's crops. According to tradition, the festival was established after Eleutherae, a town on the border between Attica and Boeotia, had chosen to become part of Attica. The Eleuthereans brought a statue of Dionysus to Athens, which was initially rejected by the Athenians. Dionysus then punished the Athenians with a plague affecting the male genitalia, which was cured when the Athenians accepted the cult of Dionysus. This was recalled each year by a procession of citizens carrying phalloi. This story relates to the original founding of the cult of Dionysus in Eleutherae in the 6th century BC, a myth that also involves the rejection, punishment, and acceptance of Dionysus

The urban festival was a relatively recent invention. This ceremony fell under the auspices of the Archons of Athens, rather than the basileus, to whom religious festivals were given when the office of archon was created in the 7th century BC.

The archon prepared for the City Dionysia as soon as he was elected, by choosing his two páredroi (πάρεδροι, "reeves", literally: "by the chair") and ten epimelētai (ἐπιμεληταί, "curators") to help organize the festival. On the first day of the festival, the pompē ("pomp", "procession") was held, in which citizens, metics, and representatives from Athenian colonies marched to the Theatre of Dionysus on the southern slope of the Acropolis, carrying the wooden statue of Dionysus Eleuthereus, the "leading" or eisagōgē (εἰσαγωγή, "introduction"). As with the Rural Dionysia, they also carried phalloi, made of wood or bronze, aloft on poles, and a cart pulled a much larger phallus. Basket-carriers and water and wine-carriers participated in the pompe here, as in the Rural Dionysia.