All countries members of the International Mathematical Union, the mother organisation of ICMI, are de facto members of ICMI. But it is also possible for a country not a member of IMU to become a member of ICMI. (Information on this matter can be obtained from the Secretary-General of ICMI.)
There are currently 92 member countries of ICMI, 70 of which are also members of IMU and 10 are associate members (am) of IMU. In the following list, (*) indicates one of the 12 members of ICMI that are not members of IMU.