Mare Liberum is a book in Latin on international law written by the Dutch jurist and philosopher Hugo Grotius, first published in 1609. In The Free Sea, Grotius formulated the new principle that the sea was international territory and all nations were free to use it for seafaring trade. The disputation was directed towards the Portuguese Mare clausum policy and their claim of monopoly on the East Indian Trade. Grotius wrote the treatise while being a counsel to the Dutch East India Company over the seizing of the Santa Catarina Portuguese carrack issue. The work was assigned to Grotius by the Zeeland Chamber of the Dutch East India Company in 1608.

Hugo Grotius, also known as Hugo de Groot or Huig de Groot, was a Dutch humanist, diplomat, lawyer, theologian, jurist, statesman, poet and playwright. A teenage prodigy, he was born in Delft and studied at Leiden University. He was imprisoned in Loevestein Castle for his involvement in the controversies over religious policy of the Dutch Republic, but escaped hidden in a chest of books that was transported to Gorinchem. Grotius wrote most of his major works in exile in France.