A treaty series is an officially published collection of treaties and other international agreements.
The League of Nations Treaty Series (LNTS) was a result of article 18 of the Covenant of the League of Nations, which stated:
Every treaty or international engagement entered into hereafter by any Member of the League shall be forthwith registered with the Secretariat and shall as soon as possible be published by it. No such treaty or international engagement shall be binding until so registered.
The League of Nations Treaty Series began publication in 1920, and was terminated in 1946, following the dissolution of the League of Nations. It contains 205 volumes with a total of 4,834 treaties. A nine-volume index has been created.
1. Every treaty and every international agreement entered into by any Member of the United Nations after the present Charter comes into force shall as soon as possible be registered with the Secretariat and published by it. 2. No party to any such treaty or international agreement which has not been registered in accordance with the provisions of paragraph 1 of this Article may invoke that treaty or agreement before any organ of the United Nations.
On December 14, 1946, the United Nations General Assembly passed resolution 97, which laid the rules for the registration of international agreements by the UN Secretariat.
The UNTS is by far the largest collection of treaties, running to over 2,900 volumes containing 158,000 treaties from 1946. Its predecessor was the League of Nations Treaty Series (LNTS).
Treaties and international agreements were formally published in Statutes at Large (Stat.) until 1948. The Department of State also published a number of collections relating specifically to treaties and other agreements:
For convenience, a number of private collections of treaties have also been published:
United Kingdom treaties and international agreements are published as command papers, along with a large number of other government documents. For ease of reference, command papers relating to treaties are given a second "Treaty Series" number, which is sometimes referred to as the United Kingdom Treaty Series (UKTS). For example, the Treaty on Open Skies was published as Cm. 5537, but also had the Treaty Series number 26 for 2002, abbreviated as  UKTS 26.
Treaties signed by the Canadian government are published in a government publication titled Canada Treaty Series.
New Zealand treaties are published in New Zealand Treaty Series (NZTS), starting publication from 1944.
The Irish Department of Foreign Affairs has published a list of all treaties of Ireland on its website, titled Irish Treaty Series. At present, all treaties from 2002 are available online.
The State of Israel has its own official collection of treaties, titled in Hebrew "Kitvey Amana" ("Treaty Writings" in Hebrew, also referred to as "Israel Treaty Series"). It includes every international agreement either signed or acceded by the Israeli government. 
The University of South Pacific in Port Vila, Vanuatu began a publication titled Pacific Islands Treaty Series, giving the texts of treaties concluded by small states in the Pacific Ocean.