The Seal of Mindaugas is a medieval seal affixed to the October 1255 act by Mindaugas, King of Lithuania, granting Selonia to the Teutonic Knights. An academic debate is ongoing to determine authenticity of the act and the seal as they might have been forged by the Knights. If it is authentic then the seal is the only surviving contemporary depiction of Mindaugas. As the most important surviving artifact from Mindaugas' times, the seal was a centerpiece of a special exhibition organized by the National Museum of Lithuania in 2003 to commemorate the 750th anniversary of Mindaugas' coronation.

Mindaugas was the first known grand duke of Lithuania and the only crowned king of Lithuania. Little is known of his origins, early life, or rise to power; he is mentioned in a 1219 treaty as an elder duke, and in 1236 as the leader of all the Lithuanians. The contemporary and modern sources discussing his ascent mention strategic marriages along with banishment or murder of his rivals. He extended his domain into regions southeast of Lithuania proper during the 1230s and 1240s. In 1250 or 1251, during the course of internal power struggles, he was baptised as a Roman Catholic; this action enabled him to establish an alliance with the Livonian Order, a long-standing antagonist of the Lithuanians. By 1245, Mindaugas was already being referred to as "the highest king" in certain documents. During the summer of 1253, he was crowned king, ruling between 300,000 and 400,000 subjects and got nicknamed as Mindaugas the Sapient by the Livonians.