The origin of alphabetic script lies in second-millennium BC Bronze Age Levantine societies. A chronological gap, however, divides the earliest evidence from the Sinai and Egypt—dated to the nineteenth century BC—and from the thirteenth-century BC corpus in Palestine. Here, the authors report a newly discovered Late Bronze Age alphabetic inscription from Tel Lachish, Israel. Dating to the fifteenth century BC, this inscription is currently the oldest securely dated alphabetic inscription from the Southern Levant, and may therefore be regarded as the ‘missing link’. The proliferation of early alphabetic writing in the Southern Levant should be considered a product of Levantine-Egyptian interaction during the mid second millennium BC, rather than of later Egyptian domination.