In 398 b.C. the alliance between Locri Epizephyrii and Syracuse is further strengthened by the marriage of Dionysius I, tyrant of Syracuse, with the maiden Doris, daughter of Xenetus, one of the most distinguished member of the Locrian aristocracy.

This event was of exceptional importance for the polis of Locri Epizephyrii; so much so that, during the first half of the fourth century b.C., the polis took considerable advantages from the military triumphs of the Syracusan ally, becoming the reference point for each of its expeditions in Magna Graecia and, especially, against Rhegion and its allies.

In return Locri Epizephyrii was always ready to answer every request for help (by sending troops and warships) made by the Syracusan ally, even during the continuous clashes between the troops of Dionysius I and the Carthaginians, eager to take control of Sicily.

Therefore, during this period Locri Epizephyrii regained some of the former glory it had in the years following the battle of the Sagra river, retaking control of the territory of Kaulon (finally destroyed by Dionysius I in 389 b.C.), of Hipponion (in 388 b.C.) and of Scylletium (between 386 b.C. and 384 b.C.); thus expanding again to the north at the expenses of Kroton's territory.

Meanwhile the army of Dionysius I captured Rhegion (in 386 b.C.), which lost its independence, falling directly under the control of Syracuse and ceasing to be a threat to Locri Epizephyrii.

With Rhegion no more a threat and with the Kroton's military strength at its end, forced to defend itself against the continuous attacks carried out by Dionysius I, Locri Epizephyrii could start again to prosper in peace.

But towards the middle of the century a new event generated great concern in the life of the Locrian polis.

During the 367 b.C. Dionysius I died and his son and successor, Dionysius II, from the father took only the name; so much so that he was driven away by his Syracusan fellow citizens in 356 b.C.

However, although Dionysius II had proven of not having the same temperament of his father, he was still the son of a woman belonging to one of the most illustrious families of the Locrian aristocracy, and due to this fact he was able to found refuge in the polis of Locri Epizephyrii.

Nevertheless after some time, eager to accumulate money to finance his return in Syracuse, he seized power establishing tyranny in Locri Epizephyrii during 352 b.C., ousting from the government the aristocracy that had always ruled Locri Epizephyrii and becoming the author of numerous vexations and atrocities against the Locrian population. Population that, truly exasperated, in 346 b.C. came up against the tyrant, slaughtering his family during his absence and preventing him from come back.

Such an event marked a turning point in the history of the polis of Locri Epizephyrii because the aristocracy lost almost all of his power in favor of a democratic system whose main bodies were a Council (boule) and an Assembly that included all the citizens (demo).

In this period Locri Epizephyrii also began to mint coin, especially considering the new importance that commerce had assumed for the polis.

In conclusion, despite the not always positive vicissitudes that took place during this age, it's possible to assert without doubts that for Locri Epizephyrii the fourth century b.C. was a period of great splendor and of a prosperity never reached before in all fields: artistic, economic and, above all, cultural. In particular, this historical period, must be remembered for figures like the poetess Nossis and the philosophers Echecrates, Timaeus and Arion, the founders of a flourishing school of Pythagoreanism (introduced in Locri at the time of Dionysius I) in which took an interest Plato itself who, according to what Cicero attests (De Finibus bonorum et malorum, V - 29, 87), went personally to Locri Epizephyrii in order to learn its basics.

But such period, unfortunately for Locri Epizephyrii, will not last long because new populations from the north (the Bruttii) threatened with ever growing insistence the Locrian territory and the ancient Syracusan ally had already lost his former power; while, at the same time, Rome expanded its borders and sought to take control over the entire Magna Graecia.