Meṣad Hashavyahu is an ancient fortress on the border of the Iron Age Kingdom of Judah facing the Philistine city of Ashdod near the Mediterranean Sea.

They might have been serving under Judean command in Josiah's battles against the military of the Twenty-sixth Dynasty of Egypt, as Yohanan Aharoni supposed.

According to Israel Finkelstein, "it is therefore quite reasonable that the unit stationed in the Egyptian fort of Messad Hashavyahu included Judahite mercenaries", and Egyptian control was more likely than Judean: "there can be little doubt that Egypt, which expanded in the late 7th century [BCE] along the coast of the Levant, was strong enough to prevent Josiah from building an isolated fort in the middle of an area in which Egypt had strong strategic interests".

The fact that the fort was south-facing may imply that it was built for the protection of Yavne and the surrounding agricultural lands, including the seaport of Yavne-Yam, against aggressors from the south, either Philistine or Egyptian.

The fortress was abandoned in 609 BCE or shortly thereafter,[3] likely associated with the loss of territory due to occupation by the Egyptian army following Josiah's death.