What major international agreement on nuclear disarmament was signed in 2025?

No new major global nuclear-disarmament treaty was signed in 2025. The question may be confusing recent discussions or diplomatic proposals with a signed international agreement.

The Treaty on the Prohibition of Nuclear Weapons is the major international treaty focused on banning nuclear weapons, but it was adopted in 2017 and opened for signature that year. The Nuclear Non-Proliferation Treaty, another central arms-control agreement, was signed in 1968.

So, unless you have a particular 2025 announcement in mind, there was no new agreement of that kind signed that year.