The Adonia (Greek: Ἀδώνια) was a festival celebrated annually by women in ancient Greece to mourn the death of Adonis, the consort of Aphrodite. It is best attested in classical Athens, though other sources provide evidence for the ritual mourning of Adonis elsewhere in the Greek world, including Hellenistic Alexandria and Argos in the second century CE.

According to Ronda R. Simms in her article, "Mourning and Community at the Athenian Adonia", the celebration of the Adonia was the only evidence that was found about worship of Adonis in Athens, as of 1997. There were no temples, statues, or priests in worship to Adonis.

In Athens, the Adonia took place annually. It is unclear when the festival was first instituted in Athens; the earliest certain references to it are in plays by Aristophanes and Cratinus in the 420s BCE, and it may be depicted in a fragment of a red-figure vase from the mid-fifth century. It was organised and celebrated by women. It was one of a number of Athenian festivals which were celebrated solely by women and addressed sexual or reproductive subjects – others included the Thesmophoria, Haloa, and Skira. Unlike these other festivals, however, the Adonia was not state-organised, or part of the official state calendar of religious celebration. In fact, it was not found to be celebrated by any official cults, like the cult of Bendis, or foreign cults, whose participants were mostly non-natives, like Isis. Prostitutes, respectable women, non-citizens and citizens alike celebrated the Adonia.

Also unlike the Thesmophoria, the Adonia was never celebrated in a designated area. Over the course of the festival, Athenian women took to the rooftops of their houses. They danced, sang, and ritually mourned the death of Adonis. They planted "Gardens of Adonis" – lettuce and fennel seeds, planted in potsherds – which sprouted before withering and dying. After the rooftop celebrations, the women descended to the streets with these Gardens of Adonis, and small images of him; they then conducted a mock funeral procession, before ritually burying the images and the remains of the gardens at sea or in springs. The rites observed during the festival are not otherwise paralleled in ancient Greek religion; like Adonis himself they probably originated in the Near East.

The date of the Adonia at Athens is uncertain, with ancient sources contradicting one another. Aristophanes, in his Lysistrata, has the festival take place in the early spring of 415 BCE, when the Sicilian Expedition was proposed; Plutarch puts the festival on the eve of the expedition's setting sail, in midsummer that year. Theophrastus' Enquiry into Plants (Περι φυτων ιστορια) and Plato's Phaedrus are both often taken as evidence for the Adonia having been celebrated in the summer. In Egypt and Syria in the Roman period, the Adonia coincided with the rising of the star Sirius in late July. As the Sicilian Expedition sailed in June 415, this contradicts both Aristophanes' and Plutarch's dating of the Adonia; the Athenian Adonia must have been celebrated at a different time.

Modern scholars disagree on which of these sources is correct. Many agree with Plutarch, and put the festival around midsummer, though Dillon argues that Aristophanes' placement of the festival near the beginning of spring is "without question" correct. Some scholars, such as James Fredal, suggest that there was in fact no fixed date for the Adonia to be celebrated.

The main feature of the festival at Athens were the "Gardens of Adonis", broken pieces of terracotta which had lettuce and fennel seeds sown in them. These seeds sprouted, but soon withered and died. Though most scholars say that these gardens withered due to being exposed to the heat of the summer, Dillon, who believes that the Adonia was held in the spring, says that the plants instead failed because they could not take root in the shallow soil held by the terracotta shards. In support of this, he cites Diogenianus, who says that in the Gardens of Adonis, seedlings "wither quickly because they have not taken root". In ancient Greece, the phrase "Gardens of Adonis" was used proverbially to refer to something "trivial and wasteful".

The symbolism of the Gardens of Adonis is also widely debated: according to James George Frazer, the Gardens of Adonis were supposed to be a sort of ritual performed in order to promote a good harvest, that the actual crops were to grow fast like the little gardens. To John J. Winkler the gardens were meant to represent how men had very little power when it came to regeneration in either plants or humans.