Konstantinos Vagianis or Kostaki Vayanis Éfendi (1846 – 1919) was a Prince of Samos between March 7, 1899, and 1900.

His sister was Princess Arapissas Vayanis, Countess Nissirios.

[1] His reign as Prince of Samos was largely a failure because he adhered to only one political party.

Despite the privileges and rights of the people of the Principality of Samos, he tried to persuade the Ottoman Sultan to abolish the decisions of the former Parliament.

For this reason, the Parliament asked for his replacement, to which the Sultan consented.