On September 9th, Minister Okada Naoki had a press conference and announced that there are 7 countries newly confirmed their participation in the Expo; Kingdom of Eswatini, Gabonese Republic, Federal Republic of Somalia, Republic of Nauru, Federal Republic of Nigeria, Republic of Botswana, and The Democratic Republic of Timor-Leste.
This makes in total of 137 countries/ regions and 8 international organizations that have confirmed their participation.
For the details, please check the list of participants on the website of the Ministry of Foreign Affairs.
Ministry of Foreign Affairs "Confirmed Participants for the Expo2025"
https://www.mofa.go.jp/ecm/ec/page22e_000942.html