Ya d'ar brezhoneg (French: Oui au breton, English: Yes to Breton) is a campaign started in the 21st century by the Ofis ar Brezhoneg ("Office of the Breton language") to promote and stimulate the use of the Breton language in daily life in Brittany, northwestern France. Breton is a Celtic language that has fallen out of general use since the mid-20th century. Efforts are underway in the region to revive the language, which is classified by UNESCO as endangered.
In the first phase, started on 5 October 2001, the office worked to engage civil society. It gained signed agreement by more than 560 enterprises and organisations to support the use of Breton in their operations.
In the second phase, started on 22 December 2004, the communes in Brittany were targeted. The office encourages communes to promote the use of Breton, for instance, by installing bilingual (Breton and French) road signs, and creating bilingual promotion posters for local events. The office has also prepared and published a road map of Brittany in Breton.