Hermaeus Soter (Ancient Greek: Ἑρμαῖος ὁ Σωτήρ, Hermaîos ho Sōtḗr, meaning "Hermaeus the Saviour") was a Western Indo-Greek king of the Eucratid dynasty, who ruled the territory of Paropamisadae in the Hindu-Kush region, with his capital in Alexandria of the Caucasus (near today's Kabul, Afghanistan).

Judging from his coins, Hermaeus' rule was long and prosperous, but came to an end when the Yuezhi, coming from neighbouring Bactria, overran most of his Greek kingdom in the Paropamisade around 70 BCE.

The chronicle tells how a king who may possibly be identified as Hermaeus received the support of the Chinese against Indo-Scythian occupants, and may explain why his kingdom was suddenly so prosperous despite the general decline of the Indo-Greeks during the period.

The Chinese general Wen Zhong, commander of the border area in western Gansu, accompanied the escort back.

They attacked Jibin (possibly with the support of the Yuezhi, themselves allies of the Chinese since around 100 BCE according to the Hanshu) and killed Wutoulao's son.

However the Yuezhi invaders imitated Hermaeus' coinage style possibly after taking over his kingdom, thereby depicting themselves as his successors.

The early Kushan ruler Heraios even imitated Hermaeus' name on his coins but misspelled it by leaving out the letter 'M'.

The first true Kushan emperor Kujula Kadphises associated himself with Hermaeus on his coins, but it is also possible that this ruler was identical to the aforementioned Heraios.