Jayanta Mongkol, the Prince Mahisara Rajaharudaya (Thai: พระองค์เจ้าไชยันตมงคล กรมหมื่นมหิศรราชหฤทัย; RTGS: Phra-ongchao Chaiyantamongkhon Krommamuen Mahisonratchaharuethai; 30 January 1866 – 15 April 1907) was a son of King Mongkut, Rama IV, and his Royal Consort Huang.

In 1897, when HM King Rama V embarked on a nine-month tour of Europe, Prince Jayanta, as the Minister of Treasury, accompanied him and saw first-hand the government administration of several European states, and the important role banks played in developing the economy and promoting international trade.

Upon returning home, the Prince assumed the task of developing international trade relations and bringing Siam's business practices in line with those in Europe.

The Prince also turned his attention to further reforming the awkward pod duang currency into a European-style metric system, involving just two units (1 baht equivalent to 100 satang).

Having succeeded with his currency reforms, Prince Jayanta went on to launch a small Siamese-owned private trust in 1904 called the Book Club.