"As my family and I have worked through the grieving process, I've said all along what I've said time and again to others, that it may very well be that that process, by the time we get through it, closes the window on mounting a realistic campaign for president," Biden said. "I've concluded it has closed."

He cited the death of his 46-year-old son, Beau, who died of brain cancer in May.

Biden made his announcement at the White House Rose Garden, standing with his wife, Jill, and President Barack Obama .

Vice President Joe Biden said Wednesday that he will not run for president, giving a boost to Hillary Clinton's White House bid.

Vice President Joe Biden (C), flanked by US President Barack Obama (L) and his wife Jill Biden (R), speaks in the Rose Garden at the White House on October 21, 2015, in Washington, DC.

Biden admitted that "there is no timetable for this process," but added that "unfortunately, I believe we're out of time — the time necessary to mount a winning campaign for the nomination."



Still, the vice president declared that he "will not be silent" during the presidential campaign.

Biden defended the Obama administration and said he's proud to have played a part in it.

"This party, (and) our nation, would be making a tragic mistake if we walk away or attempt to undo the Obama legacy," Biden said. "The American people have worked too hard and we've come too far for that."

Read More Who won the first Democratic debate?



He said his fellow Democrats should not only defend the administration's work, but they should go so far as to run their campaigns on the basis of the Obama record.

Speculation had mounted for months that Biden might enter the race.

Still, it would have been an uphill battle for Biden, who was far behind Clinton in polls. Clinton turned out a strong performance in last week's Democratic presidential debate.

The 72-year-old Biden expressed one regret about not running: "If I could be anything, I would have wanted to be the president that ended cancer."

