Isthmian Games or Isthmia (Ancient Greek: Ἴσθμια) were one of the Panhellenic Games of Ancient Greece, and were named after the Isthmus of Corinth, where they were held. As with the Nemean Games, the Isthmian Games were held both the year before and the year after the Olympic Games (the second and fourth years of an Olympiad), while the Pythian Games were held in the third year of the Olympiad cycle.
The Games were reputed to have originated as funeral games for Melicertes (also known as Palaemon), instituted by Sisyphus, legendary founder and king of Corinth, who discovered the dead body and buried it subsequently on the Isthmus. In Roman times, Melicertes was worshipped in the region.
The festival is of athletic and musical competitions to honor the sea god Poseidon. This festival is held in the spring of the second and fourth years of each Olympiad at Poseidon’s rural sanctuary on the Isthmus of Corinth in the small neck of land that connects the Peloponnesian peninsula with Central Greece. The legends have it that this festival was started with either Sisyphus, King of Corinth, or Theseus but no one is sure of who to this day. They were organized by the city Corinth until 146 BC, when Corinth was completely destroyed by the Romans. After this for a large period of time the games were held in Sicyon. In 40 BC Corinth got hold of the organization again and around 40 AD the games moved back to the Isthmos. The entrance to the temple had statues of the victors in the Isthmian games. This festival was open to all Greeks and the Isthmian games were especially popular with Athenians. The only people forbidden to attend was the inhabitants of Elis. The games were the same as those in Olympia with having wrestling, the pancratium, and horse racing. Horse racing being the most important part of the games because Poseidon was the patron of this sport. With this there was a lot of rivalry with the Athenians going against the Corinthians. The Isthmian games were also used by many as a forum for political propaganda. Since it was easy to reach both from land and sea it was an easy natural meeting place. The victor at the beginning of these events was gifted a crown of dry wild celery but it was changed to a pine wreath in Roman times. This pine became sacred to Poseidon. The victor by winning also won lofty odes, called Epinkia or triumphal odes. The Athenian who became the victor also received a public treasury which was one hundred drachmae. Although this was a fun festival for all it died out when Christianity became dominant in the 4th century AD.
Theseus, legendary king of Athens, expanded Melicertes' funeral games from a closed nightly rite into fully-fledged athletic-games event which was dedicated to Poseidon, open to all Greeks, and was at a suitable level of advancement and popularity to rival those in Olympia, which were founded by Heracles. Theseus arranged with the Corinthians for any Athenian visitors to the Isthmian games to be granted the privilege of front seats (prohedria, Ancient Greek προεδρία). Another version states that Kypselos, tyrant of Corinth in the 7th century BC, returned to the Games their old splendour.
If we are to accept the traditional date of the first Olympic Games (776 BC), we can say that the first Isthmian Games would have been held in 582 BC. 
At least until the 5th century BC (Pindar's time) the winners of the Isthmian games received a wreath of celery;  later, the wreath was altered such that it consisted of pine leaves and called Isthmian pine (Ἰσθμικὴ πίτυς).
From 228 BC or 229 BC onwards the Romans were allowed to take part in the games.
When he had arranged these things with them he went to the Isthmian games, and, the stadium being full of people, he commanded silence by trumpet and directed the herald to make this proclamation, "The Roman people and Senate, and Flamininus, their general, having vanquished the Macedonians and Philip, their king, order that Greece shall be free from foreign garrisons, not subject to tribute, and shall live under her own customs and laws." Thereupon there was great shouting and rejoicing and a scene of rapturous tumult; and groups here and there called the herald back in order that he might repeat his words for them. They threw crowns and fillets upon the general and voted statues for him in their cities. They sent ambassadors with golden crowns to the Capitol at Rome to express their gratitude, and inscribed themselves as allies of the Roman people. Such was the end of the second war between the Romans and Philip.
Since the games' inception, Corinth had always been in control of them. When Corinth was destroyed by the Romans in 146 BC, the Isthmian games continued, but were now administered by Sicyon. Corinth was rebuilt by Caesar in 44 BC. Corinth recovered ownership of the Games shortly thereafter, but they were then held in Corinth. They did not return to the Isthmus until AD 42 or 43. The Isthmian Games thereafter flourished until the mid-4th century at least. A possible late literary reference dates to 362. The circumstances of their demise are unknown. Imperial pressure against pagan rituals was heightened at the end of the 4th century, but some polytheistic cult practices certainly continued at Corinth into the 6th century.
Comparable to the Olympic Games. Among other competitions were:
Before the Games began, a truce was declared by Corinth to grant athletes safe passage through Greece. In 412 BC, even though Athens and Corinth were at war, the Athenians were invited to the games as usual.
- Pseudo-Apollodorus, Bibliotheca 3.4.3; Pausanias, Description of Greece 2.1.3, 1.44.8. It is likely that Pindar already described this version of the origin of the games (in a fragment of the Isthian odes). For more information, see E.R. Gebhard & M.W. Dickie, Melikertes-Palaimon, Hero of the Isthmian Games Archived 2007-03-10 at the Wayback Machine.
- “… the Isthmia lament Melicertes …” (… Μελικέρτην ὀδύρεται τὰ Ἴσθμια …: Eusebius, Preparation for the Gospel 2.6 (= Clemens, Protrepticus 2.34.1).
- Pausanias, Description of Greece 2.1.3, 2.2.1.
- Plutarch, Life of Theseus 25.4–5.
- Plutarch, Life of Theseus 25.4–5.
-  Archived June 1, 2007, at the Wayback Machine
- Solinus, Wonders of the World 7.14.
- According to Solinus, the Isthmian Games were constituted in the 49th Olympiad (Solinus, Wonders of the World 7.14). The 49th Olympiad began in 584 BC. The Olympic Games took place in July/August; the Isthmian Games in April/May of the second year of the Olympiad. The second year of the 49th Olympiad was from July/August 583 to July/August 582 BC. The date 582 BC is accepted by historically-derived documents, for instance, Der neue Pauly (under Isthmia).
- Ancient Greek σέλινον: Pindar, Isthmian Odes 2.16, 8.64.
- “At the Isthmus the pine, and at Nemea celery became the prize to commemorate the sufferings of Palaemon and Archemorus.” (Pausanias, Description of Greece 8.48.2).
- “As he was marching up an ascent, from the top of which they expected to have a view of the army and of the strength of the enemy, there met him by chance a train of mules loaded with parsley; which his soldiers conceived to be an ominous occurrence or ill-boding token, because this is the herb with which we not infrequently adorn the sepulchres of the dead; and there is a proverb derived from the custom, used of one who is dangerously sick, that he has need of nothing but parsley. So to ease their minds, and free them from any superstitious thoughts or forebodings of evil, Timoleon halted, and concluded an address suitable to the occasion, by saying, that a garland of triumph was here luckily brought them, and had fallen into their hands of its own accord, as an anticipation of victory: the same with which the Corinthians crown the victors in the Isthmian games, accounting chaplets of parsley the sacred wreath proper to their country; parsley being at that time still the emblem of victory at the Isthmian, as it is now at the Nemean sports; and it is not so very long ago that the pine first began to be used in its place.” “26. (1.) Ἀναβαίνοντι δ’ αὐτῷ πρὸς λόφον, ὃν ὑπερβαλόντες ἔμελλον κατ‑ όψεσθαι τὸ στράτευμα καὶ τὴν δύναμιν τῶν πολεμίων, ἐμβάλλουσιν ἡμίονοι (2.) σέλινα κομίζοντες, καὶ τοῖς στρατιώταις εἰσῆλθε πονηρὸν εἶναι τὸ ση‑ μεῖον, ὅτι τὰ μνήματα τῶν νεκρῶν εἰώθαμεν ἐπιεικῶς στεφανοῦν σελί‑ νοις· καὶ παροιμία τις ἐκ τούτου γέγονε, τὸν ἐπισφαλῶς νοσοῦντα δεῖσθαι (3.) [τοῦτον] τοῦ σελίνου. βουλόμενος οὖν αὐτοὺς ἀπαλλάξαι τῆς δεισιδαιμο‑ νίας καὶ τὴν δυσελπιστίαν ἀφελεῖν, ὁ Τιμολέων ἐπιστήσας τὴν πορείαν ἄλλα τε <πολλὰ> πρέποντα τῷ καιρῷ διελέχθη, καὶ τὸν στέφανον αὐτοῖς ἔφη πρὸ τῆς νίκης κομιζόμενον αὐτομάτως εἰς τὰς χεῖρας ἥκειν, ᾧπερ Κορίνθιοι στεφανοῦσι τοὺς Ἴσθμια νικῶντας, ἱερὸν καὶ πάτριον στέμμα (5) (4.) <τὸ> τοῦ σελίνου νομίζοντες. ἔτι γὰρ τότε τῶν Ἰσθμίων, ὥσπερ νῦν τῶν (5.) Νεμείων, τὸ σέλινον ἦν στέφανος, οὐ πάλαι δ’ ἡ πίτυς γέγονεν.” (Plutarch, Life of Timoleon).
- Todo: Oscar Broneer, ‘The Isthmian victory crown’, American Journal of Archaeology 66 (1962), pp.259–263.
- Suda, iota, 639
- Pausanias, Description of Greece 2.1.7. None of the statues have survived.
- From Solon (638–558 BC) onwards, for he laid it down that “the victor in the Isthmian games was to be paid a hundred drachmas, and the Olympic victor five hundred” (Plutarch, Live of Solon 23.3). According to Diogenes Laërtius, Solon “diminished the honours paid to Athletes who were victorious in the games, fixing the prize for a victor at Olympia at five hundred drachmae, and for one who conquered at the Isthmian games at one hundred” (Diogenes Laërtius, Lives of Philosophers 1.55: Solon Archived 2010-09-02 at the Wayback Machine; Greek). For comparison: the daily wage for a skilled worked was approximately 1 drachma. Victors in the Isthmian games were not included in those athletes that were entitled to free meals in the [Prytaneion] (IG I3 131).
- Polybius, Histories 2.12.8.
- Polybius, Histories 18.46.
- "Appian, Roman History". livius.org.
- Pausanias, Description of Greece 2.2.1.
- Pausanias, Description of Greece 2.1.2.
- Kajava, Mika (2002). "When did the Isthmian Games return to the Isthmus?". Classical Philology. 97: 168–178. doi:10.1086/449579 – via JSTOR.
- Richard M. Rothaus, Corinth: The First City of Greece (Brill, 2000), pp. 86–87.
- Pausanias, Description of Greece 5.2.4.
- Aristomache, a poetess from Erythraea, had won the prize at the Isthmian Games: … ὡς ἐν τῷ Σικυωνίων θησαυρῷ χρυσοῦν ἀνέκειτο βιβλίον Ἀριστομάχης ἀνάθημα τῆς Ἐρυθραίας ἐπικῷ … ποιήματι δὶς Ἴσθμια νενικηκυίας (Plutarch, Symposiacs/Quaestiones convivales 675b7–10 5.2 Archived 2016-01-05 at the Wayback Machine).
- 1 Corinthians 9:26
- Pausanias, Description of Greece 6.15.3.
- ”ἐς ὃ Ἰσθμικὰς σπονδὰς Κορινθίων ἐπαγγειλάντων” (Pausanias, Description of Greece 5.2.1).
- Thucydides, History of the Peloponnesian War 8.10.