Demetrio e Polibio (Italian pronunciation: [deˈmɛːtrjo e ppoˈliːbjo]; Demetrius and Polybius) is a two-act operatic dramma serio by Gioachino Rossini to a libretto by Vincenzina Viganò-Mombelli.

[1] Demetrio e Polibio was Rossini's first attempt at a full-scale opera, "assembled piecemeal"[2] during his student days at the Philharmonic Academy of Bologna in 1806.

Demetrius, holding Minteus responsible for his son's disappearance, arrives at the court of Parthia in the guise of Eumeno, a royal messenger, and demands that Siveno be turned over to Syria.

The situation starts to resolve when Eumene (Demetrius), looks at an old medallion and realizes that Siveno is actually his lost son.

Desperate at their impending separation, Lisinga tries to kill Eumene, but he finally reveals his true identity as King Demetrius and announces that Siveno is his son.