Dag Hjalmar Agne Carl Hammarskjöld ( Dag Hammarskjöld (help·info)) (29 July 1905 – 18 September 1961) was a Swedish diplomat, economist and author. He was the second Secretary-General of the United Nations. He served from April 1953 until his death in a plane crash in September 1961. He is the only person to have been awarded the Nobel Peace Prize posthumously. Hammarskjöld remains the only U.N. Secretary-General to die in office.

U.S. President John F. Kennedy called Hammarskjöld “the greatest statesman of our century.”

Dag Hammarskjöld was born in Jönköping, Sweden, but he lived most of his childhood in Uppsala. He was the fourth and youngest son of Hjalmar Hammarskjöld, prime minister of Sweden from 1914 to 1917, and Agnes Hammarskjöld (née Almquist). His ancestors had served the Swedish Crown since the 17th century. He studied first at Katedralskolan and then at Uppsala University where he graduated with a Master's degree in Political Economy and a Bachelor of Law degree. During this time he served for a year as the first Curator at Uplands Nation. He then moved to Stockholm.