Diodorus the Rhetorician is known solely from a fragmentary inscription dated to the Roman Imperial period, between the 1st and 3rd centuries CE, found in Lydia, Asia Minor. The inscription identifies him only by his title, providing no further biographical details regarding his life, education, or career.

A single oration is attributed to him, preserved in a fragmentary state on stone. The inscription captures the opening of what appears to be a rhetorical display piece. No other works are attested.

His significance derives entirely from this epigraphic evidence, which provides a material example of rhetorical culture in the Roman East. The inscribed speech fragment illustrates the public commemoration of oratory on stone monuments in provincial civic life. Diodorus thus represents the many local intellectuals whose reputations survive only through such archaeological finds.