Official EU and Japanese records show Japan entered Horizon Europe’s framework by signing an association agreement on 30 July 2026, with transitional participation already in place from 1 January 2026. That supports the basic claim. However, the status is provisionally applied pending full entry into force, and Japan’s association is not full access to every part of the programme.
Caveats
Japan’s association is not blanket participation in all of Horizon Europe; it is limited in scope, especially to Pillar II.
The agreement is being applied provisionally, so 'joined' is slightly less precise than 'signed and began provisional association.'
Some secondary and social-media sources describe the development more broadly than the underlying legal documents do.