Lassa Francis Lawrence Oppenheim (30 March 1858 – 7 October 1919) was a German jurist. He has been characterized as the father of the modern discipline of international law, especially the hard legal positivist school of thought. His two-volume International Law: A Treatise has influenced international law.^[1] He inspired Joseph Raz and Prosper Weil.

Oppenheim was born in Windecken near the Free City of Frankfurt, German Confederation, the son of a Jewish horse trader,^[2] and educated at the Universities of Berlin, Göttingen and Heidelberg.^[3]

In 1881, he obtained his PhD of Law at the University of Göttingen. In 1883, he went to the University of Leipzig, where he became a disciple of the renowned Professor of Criminal Law Karl Binding. In 1885 he completed his Habilitation at the University of Freiburg and taught criminal law there until he moved to the University of Basel in 1892. In Basel, Oppenheim still worked on criminal law. It was not until he moved to the United Kingdom that he turned from criminal law to international law.

Oppenheim moved to the United Kingdom in 1895, acquiring citizenship in 1900, and lived there until his death.

He first lectured at the London School of Economics and in 1908 became the Whewell Professor of International Law in the University of Cambridge. He is the author of the internationally renowned International Law: A Treatise, the first edition of which was published in 1905–1906.

The eighth edition of the part on peace was edited by Sir Hersch Lauterpacht; the ninth and most recent edition of the same part was co-edited by Sir Robert Yewdall Jennings and Sir Arthur Watts. The work is still considered a standard text of international Law.^[4]