Jarmuth (or Yarmuth) is located about 1 1/4 miles north of the Valley of Elah, and 5 miles south of Beth-shemesh and the Sorek Valley. The site is mentioned 6 times in the book of Joshua (10:3, 5, 23; 12:11; 15:35), and in Nehemiah 11:29). The name is used in Joshua 21:29, but the Jarmuth mentioned there seems to be a town in the territory of Issachar.
Jarmuth was a Canaanite city conquered by the Israelites in the days of Joshua. It became part of the kingdom of Judah.
Michael Avi-Yonah says,
It has been identified with Khirbat al-Yarmūk (Eusebius calls it Iermochus), a large and prominent mound east of Kafr Zakariyya where surveys have revealed a large city surrounded by a massive stone wall from the Early Bronze Age and a smaller but higher mound containing pottery ranging from the Late Bronze to Byzantine periods. (Encyclopaedia Judaica)
Some excavations were conducted in the 1980s by Pierre De Miroschedji. The excavator says,
Given its size and the density of its construction, the EB III [about 2300 B.C.] city of Jarmuth may have had a population of about 3,000, engaged mainly in agriculture (cereals, vegetables, grapes, and especially olives) and animal husbandry (mostly sheep and goats, cattle and donkeys being used for traction and transport). (The Anchor Bible Dictionary 3:646)
The photo below was taken from Khirbet Qeiyafa, 1 1/4 miles south of Jarmuth.