Isernia (Italian pronunciation: [iˈzɛrnja] ^ⓘ) is a town and comune in the southern Italian region of Molise, and the capital of the province of Isernia.

Situated on a rocky crest rising from 350 to 475 metres (1,148 to 1,558 ft) between the Carpino and the Sordo rivers, the plan of Isernia still reflects the ancient layout of the Roman town, with a central wide street, the cardo maximus, still represented by Corso Marcelli, and side streets at right angles on both sides.

The comune of Isernia includes 16 frazioni. The most densely populated is Castelromano which is positioned in a plain at the base of the La Romana mount, elevation 862 metres (2,828 ft), 5 kilometres (3 mi) from Isernia.

The city's Roman name, Aesernia, reflects probably a former Samnite toponym, but a connection to an Indo-European root, aeser, which means "water", is tenuous.

Classical Aesernia was a city of Samnium, included within the territory of the Pentri tribe, situated in the valley of the Vulturnus (modern Volturno), on a small stream flowing into that river, and distant 22 kilometres (14 mi) from Venafrum (modern Venafro). The Itinerary (in which the name is written "Serni") places it on the road from Aufidena to Bovianum, at the distance of 45 kilometres (28 mi) from the former, and 29 kilometres (18 mi) from the latter; but the former number is corrupt, as are the distances in the Tabula Peutingeriana.

The first mention of it in history occurs in 295 BC, at which time it had already fallen into the hands of the Romans, together with the whole valley of the Vulturnus. After the complete subjugation of the Samnites, a colony, with Latin rights (colonia Latina) was settled there by the Romans in 264 BC the city, a key communication center between southern Italy and the inner Appennine Regions. This colony is again mentioned in 209 BC as one of the eighteen which remained faithful to Rome at the most trying period of the Second Punic War. During the Social War it adhered to the Roman cause, and was gallantly defended against the Samnite general Vettius Scato, by Marcus Claudius Marcellus, nor was it till after a long protracted siege that it was compelled by famine to surrender, 90 BC. Henceforth it continued in the hands of the confederates; and at a later period of the contest afforded a shelter to the Samnite leader, Gaius Papius Mutilus, after his defeat by Lucius Cornelius Sulla. It even became for a time, after the successive fall of Corfinium (modern Corfinio) and Bovianum, the headquarters of the Italic League. At this time it was evidently a place of importance and a strong fortress, but it was so severely punished for its defection by Sulla after the final defeat of the Samnites in 88 BC, that Strabo speaks of it as in his time utterly deserted.

We learn, however, that a colony was sent there by Julius Caesar, and again by Augustus; but apparently with little success, on which account it was recolonized under Nero. It never, however, enjoyed the rank of a colony, but appears from inscriptions to have been a municipal town of some importance in the time of Trajan and the Antonines. To this period belong the remains of an aqueduct and a fine Roman bridge, still visible; while the lower parts of the modern walls present considerable portions of polygonal construction, which may be assigned either to the ancient Samnite city, or to the first Roman colony. The modern city is still the see of a bishop. The massively constructed podium now underlying the cathedral probably supported the Capitolium.

In the early 7th century AD, what are today the comuni of Isernia as well as Bojano and Sepino were the places where Grimoald I of Benevento settled a group of Bulgars, seeking refuge from the Avars; the Bulgars were for many generations a distinctive part of the population, until finally assimilated in their Italian environment (see Bulgarians in Italy, Old Great Bulgaria#Bulgars in Southern Italy).