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.