Nikolaj Frederik Severin Grundtvig (Danish: [ˈne̝kolɑjˀ ˈfʁeðˀʁek ˈse̝vəʁiˀn ˈkʁɔntvi, - ˈfʁæðˀʁæk -]; 8 September 1783 – 2 September 1872), most often referred to as N. F. S. Grundtvig, was a Danish pastor, author, poet, philosopher, historian, teacher and politician. He was one of the most influential people in Danish history, as his philosophy gave rise to a new form of nationalism in the last half of the 19th century. It was steeped in the national literature and supported by deep spirituality.

Grundtvig holds a unique position in the cultural history of his country. Grundtvig and his followers are credited with being very influential in the formulation of modern Danish national consciousness. He was active during the Danish Golden Age, but his style of writing and fields of reference are not immediately accessible to a foreigner, thus his international importance does not match that of his contemporaries Hans Christian Andersen and Søren Kierkegaard.

Called Frederik rather than Nikolaj by those close to him. N. F. S. Grundtvig was the son of a Lutheran pastor in Udby, Johan Ottosen Grundtvig (1734–1813) and was born there. He was brought up in a very religious atmosphere, although his mother also had great respect for old Norse legends and traditions. He was schooled in the tradition of the European Enlightenment, but his faith in reason was also influenced by German romanticism and the ancient history of the Nordic countries.

In 1791 he was sent to Thyregod in Central Jutland to live and study with pastor Laurids Svindt Feld (1750–1803). He subsequently studied at the Aarhus Katedralskole, the cathedral school of Aarhus, from 1798 until graduation. He left for Copenhagen in 1800 to study theology and was accepted to the University of Copenhagen in 1801. At the close of his university life, Grundtvig began to study Icelandic and the Icelandic Sagas.

In 1805, Grundtvig took a position as tutor in a house on the island of Langeland. The next three years he used his free time to study writers Shakespeare, Schiller, and Fichte. In 1802, his cousin, the philosopher Henrich Steffens, returned to Copenhagen full of the teaching of Friedrich Wilhelm Joseph Schelling. His lectures and the early poetry of Adam Oehlenschläger opened Grundtvig's eyes to the new era in literature. His first work, On the Songs in the Edda, attracted no attention.

Returning to Copenhagen in 1808, Grundtvig achieved greater success with his Northern Mythology, and again in 1809 with a long drama, The Fall of the Heroic Life in the North. Grundtvig boldly denounced the clergy of the city in his first sermon in 1810. When Grundtvig published the sermon three weeks later it offended the ecclesiastical authorities, and they demanded him punished.

In 1810, Grundtvig underwent a religious crisis and converted to a strongly held Lutheranism. He retired to his father's country parish in Udby as his chaplain. His new-found conviction was expressed in his The First World Chronicle (Kort Begreb af Verdens Krønike i Sammenhæng) of 1812, a presentation of European history in which he attempted to explain how belief in God has been viewed throughout human history and in which he criticized the ideology of many prominent Danes. It won him notoriety among his peers and cost him several friends, notably the historian Christian Molbech. Upon his father's death in 1813, Grundtvig applied to be his successor in the parish but was rejected.

In the following years his rate of publication was staggering: aside from a continuing stream of articles and poems, he wrote a number of books, including two more histories of the world (1814 and 1817); the long historical poem Roskilde-Riim (Rhyme of Roskilde; 1813); and a book-sized commentary, Roskilde Saga. From 1816 to 1819 he was editor of and almost sole contributor to a philosophical and polemical journal entitled Danne-Virke, which also published poetry.