Al-Mu'tassimu Billahi Muhibbuddin Sultan Abdul Halim Mu'adzam Shah Al-Haj ibni Almarhum Sultan Badlishah, DMN GCB KStJ (born 28 November 1927) is the current Sultan of Kedah, reigning since 1958. He served as the fifth Yang di-Pertuan Agong of Malaysia from 1970 to 1975, and as the 14th Yang di-Pertuan Agong from 2011 to 2016. He is the first person to reign as Yang di-Pertuan Agong twice, as well as the oldest elected to the office. He is also the second longest-reigning living monarch in the world and the second-oldest currently reigning monarch after Queen Elizabeth II of the United Kingdom.

Born at Istana Anak Bukit near Alor Star as Tunku Abdul Halim, he was the second, but eldest surviving son, of Sultan Badlishah (1894-1958; reigned 1943–1958), who later became the 27th Sultan of Kedah. Of Malay and Thai descent, his mother was the Kedah-born princess Tunku Sofiah binti Tunku Mahmud (born 1899), who died in an automobile accident on 28 February 1934. Abdul Halim's maternal grandfather, Tunku Mahmud, was once Raja Muda (heir presumptive) to the throne of Kedah.

He was educated at Alor Merah and Titi Gajah Malay schools and Sultan Abdul Hamid College in Alor Star between 1946 and 1948. He went on to Wadham College, Oxford and obtained a Diploma in Social Science and Public Administration. He subsequently joined the Kedah Administrative Service, serving in the Alor Star district office and later, the state treasury.

On 6 August 1949 Abdul Halim was appointed Raja Muda or heir apparent, and acceded as the twenty-seventh Sultan of Kedah on his father's death on 15 July 1958. He was installed at the Balai Besar, Kota Star Palace in Alor Star on 20 February 1959, in a ceremony not held since 1710.

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