On 25 June 2017 the Belgian parliament approved of a gender recognition law that enables the change of gender marker and name in a simple administrative procedure before the civil registry. The procedure includes a waiting period of three months. After this time the applicant has to confirm their intention to change their officially registered gender marker. The procedure is open for people from age 16. Minors though have to bring along also a medical expert statement. The law has come into effect on 1 January 2018. Information for how to change personal records in Belgium can be found in the information leaflet that the Belgian government is distributing in this pdf.

The text of the law can be found here in Dutch | French | English (google translate)