The first new Marines will join the fight by Christmas.
The escalation - to be completed by next summer - is designed to reverse significant Taliban advances since Obama took office 10 months ago and to fast-track the training of Afghan soldiers and police toward the goal of hastening an eventual US pullout.
The size and speed of the troop increase will put a heavy strain on the military, which still maintains a force of more than 100,000 in Iraq and already has 68,000 in Afghanistan.
"The 30,000 additional troops that I am announcing tonight will deploy in the first part of 2010 at the fastest pace possible, so that they can target the insurgency and secure key population centres," Obama announced.
The increased troops, Obama said, "will increase our ability to train competent Afghan security forces, and to partner with them so that more Afghans can get into the fight. And they will help create the conditions for the United States to transfer responsibility to the Afghans."
Looking to America's experience in Iraq, Obama said a US withdrawal would be executed "responsibly, taking into account conditions on the ground."
"We will continue to advise and assist Afghanistan's security forces to ensure that they can succeed over the long haul. But it will be clear to the Afghan government and, more importantly, to the Afghan people that they will ultimately be responsible for their own country," Obama said.
Obama also leaned heavily on NATO allies and other countries to join in escalating the fight.
"We must come together to end this war successfully," the president said. "For what's at stake is not simply a test of NATO's credibility. What's at stake is the security of our allies, and the common security of the world."
Obama's Tuesday evening (Wednesday 1pm NZT) speech to cadets at the US Military Academy at West Point, New York ends three months of exacting deliberations that won praise from supporters and criticism from opponents.
Former Vice President Dick Cheney said Obama was "dithering," too inexperienced to make a decision on the troop buildup requested in September by commanding General Stanley McChrystal.
Senior officials said Obama also would underscore his commitment to stabilising Afghanistan and scouring corruption out of the government of President Hamid Karzai.
Obama has vowed to prevent Afghanistan from again becoming a safe haven for al-Qaida boss Osama bin Laden and his terrorist organisation.
Most of the new forces will be combat troops. Military officials said the Army brigades most likely to be sent will come from Fort Drum in New York and Fort Campbell in Kentucky. Marines, who will be the vanguard, will most likely come primarily from Camp Lejeune in North Carolina.
There will be about 5000 dedicated trainers in the 30,000, showing the emphasis on preparing Afghans to take over their own security. And the president is making clear to his generals that all troops, even if designated as combat, must consider themselves trainers.
Announcing a start to a US withdrawal by July 2011 does not tie the United States to an "end date" for the war, officials said.
The address could become a defining moment of the Obama presidency, a political gamble that may weigh heavily on his chances for a second White House term.
It represents the beginning of a sales job to restore support for the war effort among an American public grown increasingly pessimistic about success - and among some fellow Democrats in Congress wary of or even opposed to spending billions more dollars and putting tens of thousands more US soldiers and Marines in harm's way.