Kremlin spokesman Dmitry Peskov said the New START treaty, which set limits on each side's missiles, launchers and strategic warheads, would lapse at the end of Thursday, Reuters reported.
Arms control experts had previously said their assumption was that it expired at the end of Wednesday.
"Today the day will end, and it (the treaty) will cease to have any effect," Peskov told reporters.
Russia had suggested both sides voluntarily extend the terms of the agreement for one year to provide time to discuss a successor treaty, a proposal which it said the United States had never formally answered.
"The agreement is coming to an end. We view this negatively and express our regret," said Peskov, who said the matter had come up in a call between Russian President Vladimir Putin and Chinese President Xi Jinping a day earlier.
"What happens next depends on how events unfold. In any case, the Russian Federation will maintain its responsible and attentive approach to the issue of strategic stability in the field of nuclear weapons and, of course, as always, will be guided first and foremost by its national interests."