Hans Lassen Martensen (August 19, 1808 – February 3, 1884) was a Danish bishop and academic.
Martensen was born in a middle-class Lutheran family in Flensburg, in the Duchy of Schleswig (now Germany), as their only son. At that time Schleswig was a duchy between Holstein and Denmark. He grew up in a German-speaking society, while his father who was a schoolmaster, writer and sea-captain preferred to use Danish. Consequently the young Martensen upbrought in a multicultural situation and reconciliation of different cultures became his one of central interests through his life.
He studied theology in Copenhagen, and later was ordained in the Danish Church. From 1834 till 1836 he travelled foreign countries. He visited several cities including Berlin, Munich, Vienna and Paris. He met influential intellects including Hegel, David Strauss, Franz von Baader and Schelling. During his travel, he began to read mystics, Eckhart, Tauler and Boehme. Also he studied Dante's The Divine Comedy with a great interest.
At Copenhagen he was lektor in theology in 1838, professor extra-ordinarius in 1840, court preacher also in 1845, and professor ordinarius in 1850. Once he was offered to a bishopric from Swedish Church but declined. In 1854 however he gave up his educational career and was made bishop of Zealand, the Danish Primate. In his studies he had come under the influence of Schleiermacher, Hegel and Franz von Baader; but he was a man of independent mind, and developed a peculiar speculative theology which showed a disposition towards mysticism and theosophy.
His contributions to theological literature included treatises on Christian ethics and dogmatics, on moral philosophy, on baptism, and a sketch of the life of Jakob Boehme, who exercised so marked an influence on the mind of the great English theologian of the 18th century, William Law. Martensen was a distinguished preacher, and his works were translated into various languages. The "official" eulogy he pronounced upon Bishop Jacob Peter Mynster (1775–1854) in 1854, in which he affirmed that the deceased man was one of the authentic truth-witnesses of Christianity to have appeared in the world since apostolic times, brought down upon his head the invectives of the philosopher Søren Kierkegaard.