Dr. Daniel G. Costelloe

Daniel Costelloe is a counsel in the Litigation/Controversy Department and is a member of the International Arbitration Practice Group. Dr. Costelloe focuses his practice on international arbitration matters across a range of industries, including oil and gas, financial services, manufacturing and automotive. He has represented clients in arbitrations under a variety of arbitral rules and has particular experience with the ICSID, ICSID Additional Facility, LCIA, UNCITRAL and ICC arbitration rules. Recent highlights include representing investors in three investment-treaty cases against eastern European states and representing a government in a complex contractual dispute governed by English law with upstream oil and gas investors. In addition, he advises clients on public international law issues and in arbitration-related court proceedings. Dr. Costelloe also has experience advising on countermeasures in connection with disputes before the World Trade Organization.

Dr. Costelloe also acts in pro bono matters. Highlights include working with the Media Legal Defence Initiative on a challenge before the East African Court of Justice to a national law's compatibility with a state's treaty obligations and on an application to the European Court of Human Rights.

Dr. Costelloe publishes on various topics within the field of public international law, including the law of treaties, international responsibility and state succession. He is the author of Legal Consequences of Peremptory Norms in International Law (Cambridge University Press, 2017) and a co-editor of the Oxford Handbook of Jurisdiction in International Law (Oxford University Press, 2019). He has taught undergraduate supervisions in public international law at Cambridge University and guest classes on the law of treaties at Queen Mary University of London, and he regularly speaks at conferences and expert seminars on current issues in international law.

Professional Activities

  • American Society of International Law
  • Chatham House, Royal Institute of International Affairs


  • Featured as a leading arbitrator in the 2020–2021 editions of Who's Who Legal: Arbitration – Future Leaders with sources saying he is "brilliant" and "dedicated"
  • Recognized in Euromoney's LMG Rising Stars Expert Guide 2020 in the categories of commercial arbitration and United Kingdom
  • Recognized in the 2018 edition of The Legal 500 UK
  • Awarded the 2016 ICLQ Young Scholar Prize by the Editorial Board of the International and Comparative Law Quarterly for his article "Treaty Succession in Annexed Territory"

Insights & News


  • Education

    • PhD, Law, University of Cambridge, 2014

      WM Tapp Studentship in Law, Managing Editor and Blog Editor, Cambridge Journal of International and Comparative Law
    • LLM, Harvard Law School, 2010

      Line Editor, Harvard International Law Journal
    • LLB (Law with French Law), University College London, 2009

      first class Andrews Medal, Hurst Prize, Richardson Prize
  • Admissions

    • New York

    • District of Columbia

  • Languages

    • German

    • French