Belgian lawyer and bibliographer who, with Henri-Marie Lafontaine (Lafontaine, Henri-Marie), devised the Universal Decimal Classification (UDC) system of subject groups for library collections.

Otlet was educated at the University of Louvain and at the Free University of Brussels, where he earned a law degree in 1890. Bored with law and absorbed with books and libraries, he found employment at the new Société des Études Sociales et Politiques. He soon became friends with Lafontaine, who was also a lawyer and bibliophile. Together they were instrumental in forming the first international bibliographic conference and the Institut International de Bibliographe (later the Fédération Internationale de Documentation 【FID】).

Both Otlet and Lafontaine saw the need for an international index for documentation. Strongly influenced by the Dewey Decimal System of Classification developed by Melvil Dewey (published in 1876), the two men eventually developed the UDC. First published in 1899, the UDC appeared in French as Manuel du répertoire universel bibliographique (Manual of the Universal Library Index). A second edition of the index appeared in the years 1927–33 under the title Classification décimale universelle (Universal Decimal Classification). The system is the classification scheme most widely used on the European continent. Translation of the UDC into English was begun in 1931; an abridged English-language edition appeared in 1948. Otlet's Traité de la documentation (Treatise on Documentation) appeared in 1934.