According to a report by BBC News, a teenage volunteer found the shard during a dig about 20km (12 miles) south-west of Jerusalem. Experts at Hebrew University said that dating showed it was written 3,000 years ago - about 1,000 years earlier than the Dead Sea Scrolls. Other scientists cautioned that further study was needed to understand it.
Preliminary investigations since the shard was found in July have deciphered some words, including 'judge', 'slave' and 'king'.
The characters are written in proto-Canaanite, a precursor of the Hebrew alphabet.
Lead archaeologist Yosef Garfinkel identified it as Hebrew because of a three-letter verb meaning "to do", which he said was only used in Hebrew.
"That leads us to believe that this is Hebrew, and that this is the oldest Hebrew inscription that has been found," he said.
The shard and other artefacts were found at the site of Khirbet Qeiyafa, overlooking the Valley of Elah, where the Bible says the Israelite David fought the Philistine giant Goliath.
Garfinkel said that the findings could shed significant light on the period of King David's reign.
"The chronology and geography of Khirbet Qeiyafa create a unique meeting point between the mythology, history, historiography and archaeology of King David," he said.
According to Hebrew University archaeologist Amihai Mazar, the inscription was "very important", as it is the longest proto-Canaanite text ever found.