The International Mathematical Union (IMU) is an international organization devoted to international cooperation in the field of mathematics across the world. It is a member of the International Science Council (ISC) and supports the International Congress of Mathematicians (ICM). Its members are national mathematics organizations from more than 80 countries.

The objectives of the International Mathematical Union are: promoting international cooperation in mathematics, supporting and assisting the International Congress of Mathematicians and other international scientific meetings/conferences, acknowledging outstanding research contributions to mathematics through the awarding of scientific prizes, and encouraging and supporting other international mathematical activities, considered likely to contribute to the development of mathematical science in any of its aspects, whether pure, applied, or educational.

The IMU was established in 1920, but dissolved in September 1932 and reestablished in 1950 at the Constitutive Convention in New York, de jure on September 10, 1951, when ten countries had become members. The last milestone was the General Assembly in March 1952, in Rome, Italy where the activities of the new IMU were inaugurated and the first Executive Committee, President and various commissions were elected. In 1952 the IMU was also readmitted to the ICSU. The past president of the Union is Carlos Kenig (2019–2022). The current president is Hiraku Nakajima.

At the 16th meeting of the IMU General Assembly in Bangalore, India, in August 2010, Berlin was chosen as the location of the permanent office of the IMU, which was opened on January 1, 2011, and is hosted by the Weierstrass Institute for Applied Analysis and Stochastics (WIAS), an institute of the Gottfried Wilhelm Leibniz Scientific Community, with about 120 scientists engaging in mathematical research applied to complex problems in industry and commerce.

IMU has a close relationship to mathematics education through its International Commission on Mathematical Instruction (ICMI). This commission is organized similarly to IMU with its own Executive Committee and General Assembly.

Developing countries are a high priority for the IMU and a significant percentage of its budget, including grants received from individuals, mathematical societies, foundations, and funding agencies, is spent on activities for developing countries. Since 2011 this has been coordinated by the Commission for Developing Countries (CDC).

The Committee for Women in Mathematics (CWM) is concerned with issues related to women in mathematics worldwide. It organizes the World Meeting for Women in Mathematics ${\textstyle ((\mathrm {WM} )^{2})}$ as a satellite event of ICM.

The International Commission on the History of Mathematics (ICHM) is operated jointly by the IMU and the Division of the History of Science (DHS) of the International Union of History and Philosophy of Science (IUHPS).