Sultan Yahya Petra ibni Almarhum Sultan Ibrahim GCMG (Jawi: سلطان يحيى ڤيترا ابن المرحوم سلطان إبراهيم‎; 10 December 1917 – 29 March 1979) was Sultan of Kelantan from 1960, and the sixth Yang di-Pertuan Agong, the constitutional monarch of Malaysia, from 1975, until his death in 1979.

He was born on 10 December 1917, as Tengku Yahya Petra at Istana Balai Besar in Kota Bharu, Kelantan. He was the second son of Sultan Ibrahim (b. 1897; reigned 1944–1960) by his third wife, Cik Embong binti Encik Daud (1899–1971).^[1]

The young Tengku Yahya Petra was raised by his childless uncle, Tengku Ismail (later Sultan Ismail). He was sent to the Francis Light School in Penang before continuing his studies in England. His uncle, Sultan Ismail, appointed him Tengku Temenggong Kelantan on 21 July 1939. He was later promoted to Tengku Bendahara Kelantan on 6 February 1945 by his father, Sultan Ibrahim. He served in various Kelantan civil service posts from 1941 to 1948.^[2]

Tengku Indra Petra was the eldest son of Sultan Ibrahim and elder brother of Sultan Yahya Petra. After Sultan Ibrahim succeeded his childless brother Sultan Ismail, Tengku Indra Petra was appointed heir apparent with the title of Raja Muda Kelantan on 25 October 1944. However, due to conflict with his father, he was dismissed from the post and removed from the line of succession by his father's decree on 1 February 1948. On the same day, Sultan Yahya Petra replaced his brother as heir apparent with the new title of Tengku Mahkota Kelantan.

Tengku Indra Petra became a politician and was elected a Member of Parliament (MP) in the first federal legislative election of 1955. Tengku Indra's descendants have since disputed their family's exclusion from the line succession of the Kelantan throne.^[3]

Tengku Indra Petra did not preside over the installation of Sultan Yahya Petra's successor, Sultan Ismail Petra, It was Tengku Panglima Raja Tengku Ahmad who presided over installations of Sultan Ibrahim, Sultan Yahya Petra and Sultan Ismail Petra. Tengku Panglima Raja was the father of the former Sultanah of Johor, Sultanah Zanariah binti Tengku Ahmad.

Sultan Yahya Petra (as he became) succeeded his father a day after the latter's death on 9 July 1960. He was crowned on 17 July 1961 at Istana Balai Besar in Kota Bharu.

Sultan Yahya Petra served as Deputy Yang di-Pertuan Agong, the federal deputy king between 21 September 1970 and 20 September 1975.

During the election of the sixth Yang di-Pertuan Agong (the federal king), the most senior ruler Sultan Ismail of Johor declined to be considered. Sultan Yahya Petra also declined nomination at first due to having suffered a serious stroke, but changed his mind and was duly elected.^[4] His term began from 21 September 1975.

Malaysia's second prime minister Tun Abdul Razak died on 14 January 1976 less than four months into Sultan Yahya Petra's reign as Yang di-Pertuan Agong.

In 1977 a state of emergency was declared in his own state following a political crisis and violence.

Sultan Yahya Petra died of an apparent heart attack at 3:45 pm at the National Palace on 29 March 1979. His coffin lay in state at the National Palace for a day and was then taken by plane to Kota Bharu where it was buried at the Kelantan Royal Mausoleum.^[5]

He was married to Tengku Zainab binti Tengku Sri Utama Raja Tengku Muhammad Petra (1917–1993), who was styled Raja Perempuan Zainab II (her stepmother-in-law was Raja Perempuan Zainab I, consort of Sultan Ibrahim) and Raja Permaisuri Agong. Sultan Yahya Petra and Raja Perempuan Zainab II had one son and six daughters. However, their two daughters died when they were young. He also married Tengku Alexandria binti Tengku Yusuf and was given a son.^[6][7][8]

Sultan Yahya Petra held the rank of Marshal of the Royal Malaysian Air Force.^{[citation needed]}