This is the tomb of Şehzade (Prince) Osman, a son of Şehzade Ahmet, the second son of sultan Beyazid II. It was built in 1513.

Şehzade Ahmet (being the sultan’s oldest living son, and governor of Amasya at the time) waged war against his younger brother Selim, who had become sultan (Selim I) after forcing their father to abdicate (1512). Finally, the forces of Selim and Ahmet fought a battle near Yenişehir, Bursa on April 24, 1513. Ahmet's forces were defeated; he was arrested and executed shortly after. In this chain of events, Selim I ordered Ahmet’s son Osman to be put to death too, which happened in Amasya the same year.

Correspondent: J.M.Criel, Antwerpen.
Sources: ‘Vakıf Abideler ve eski Eserler’ - Vakıflar Genel Müdürlüğü, Ankara 1983 & Wikipedia.