The US has extended the New Start nuclear arms control treaty with Russia for five years.
Announcing the move, Secretary of State Antony Blinken said it made the world safer.
President Joe Biden and his Russian counterpart Vladimir Putin reportedly discussed it by phone a week ago, and Mr Putin signed it into law on Friday.
The treaty had been set to lapse on 5 February, as the Trump administration had refused to approve the extension. It had sought tougher verification procedures and some inclusion of tactical weapons, as well as an expansion of the deal to cover China.