Unique Master Citizen Number (Serbo-Croatian: Jedinstveni matični broj građana / Јединствени матични број грађана, JMBG / ЈМБГ, Macedonian: Единствен матичен број на граѓанинот, ЕМБГ, Slovene: enotna matična številka občana, EMŠO) is an identification number that was assigned to every citizen of former Yugoslav republics of the SFR Yugoslavia. It continues to be used in almost all of the countries that were created after the dissolution of Yugoslavia – Bosnia and Herzegovina, Montenegro, North Macedonia, Serbia and Slovenia use it in its original form, while Croatia has switched to a new identification number called the Personal Identification Number (Osobni identifikacijski broj, OIB).

The JMBG was introduced in the Socialist Federal Republic of Yugoslavia on January 8, 1977 and applied to all citizens born before then and alive at the time. All six republics passed a law on the Unique Master Citizen Number.

Although the Republic of Croatia continued to use the JMBG after gaining independence in 2002 the official name of the number there was changed to Matični broj građana (Master Citizen Number), acronym MBG. Advocates of the right to privacy argued that JMBG was a piece of personally identifiable information that needed to be protected by information privacy law, mostly because it was unique and it included the person's date of birth. When the law to that effect was passed in 2003 JMBG was removed from identity cards, driver's licenses and similar documents. In 2009, Croatia passed a new law that introduced a different unique identifier called the Personal Identification Number (Croatian: Osobni identifikacijski broj, OIB). The OIB consists of 11 randomly chosen digits and has been assigned to all Croatian citizens, companies registered in Croatia and foreign nationals residing in Croatia. Although the OIB is in use, the MBG law remains in effect, and the MBG number is still issued. MBG no longer appears on Croatian identity cards since 2003, instead OIB does, since 2013.

In Bosnia and Herzegovina in 2001 the official name of the number was changed to Jedinstveni matični broj (Unique Master Number), although acronym JMBG is still in use.

The number is made up of 13 digits in a form "DD MM YYY RR BBB K" (whitespaces are for convenience; digits are written without separation) where:

The checksum is calculated from the mapping DDMMYYYRRBBBK = abcdefghijklm, using the formula:

Note: there has been a small number of JMBGs that were assigned by valid authorities but which had an invalid checksum. Also, there are a few duplicate JMBGs in existence. The common anecdotal explanation for these is simple operator error. Reportedly these mistakes happened more often in the early 1990s. The chances of running into exceptions are reportedly low, and whether such exceptions justify questioning the use of JMBG as a unique identifier has not been scientifically analyzed.^{[citation needed]}

As an example, a valid identification number is 0101006500006; it is the number of the first male baby registered in Slovenia on January 1, 2006.