The inscriptions are thought to have been made by the Olmecs, an ancient pre-Columbian people known for creating large statues of heads.
The stone slab, named the "Cascajal block", was first uncovered by road builders digging up an ancient mound at Cascajal, outside San Lorenzo, in the late 1990s.
It weighs about 12kg (26lbs) and measures 36cm (14in) in length, 21cm (8in) in width and 13cm (5in) in thickness. Its text consists of 62 signs, some of which are repeated up to four times.
BBC NEWS | Science/Nature | 'Oldest' New World writing found