The Global Intermediary Identification Number (GIIN) is a unique identification number assigned to foreign entities that are foreign financial institutions (FFI), financial institution branches, direct reporting non-financial foreign entities (NFFE), sponsoring entities, sponsored entities, and sponsored subsidiary branches.
The GIIN is assigned once the entity registers through the FATCA registration system on the IRS website. Foreign entities need the GIIN to file Form 8966 (FATCA Report) with the IRS.
The GIIN is a 19-character number comprising several identifiers and has the following format: XXXXXX.XXXXX.XX.XXX
The IRS website has more details on the composition of the GIIN. The IRS maintains a searchable public database which lists all of the GIINs, Financial Institution Names, and Country of Registration.