Romans may have used 20-Sided die almost two millennia before D&D, but people in ancient Egypt were casting icosahedra even earlier. Pictured above is a twenty-faced die dating from somewhere between 304 and 30 B.C., a timespan also known as Egypt's Ptolemaic Period.
According to The Metropolitan Museum of Art, where the gamepiece is held, the die was once held in the collection of one Reverend Chauncey Murch, who acquired it between 1883 and 1906 while conducting missionary work in Egypt.
Got a 20-sided die that predates the Ptolemaic Period? Post about it in the comments.