Trump's plans were finalised during a presidential retreat with top advisers at Camp David on Saturday.
"Many decisions made, including on Afghanistan," Trump wrote on Twitter after the meeting.
Trump did not specify how many more troops will be sent to Afghanistan, but congressional officials said the Administration has told them it will be about 4000.
An increase in the 8500 US service members currently in Afghanistan represents a bow to reality for a president who, before taking office, called for a "speedy withdrawal" from a war that began in the aftermath of the September 11, 2001, terrorist attacks.
As a candidate, Trump denounced Afghanistan as a "total disaster" and railed that the costly conflict in Central Asia drained enormous resources at a time of more pressing needs at home for American taxpayers.
In his speech, Trump said his calculus as president is different than it was as a candidate.
"My original instinct was to pull out, and historically I like following my instincts," Trump said. "But all my life, I've heard that decisions are much different when you sit behind the desk in the Oval Office."
After taking office, Trump announced that he would delegate authority to the Pentagon to set troop levels. That raised fears among some lawmakers and foreign policy analysts that even a modest initial increase could escalate rapidly and plunge the United States more deeply back into a conflict that already has resulted in the deaths of 2403 Americans.
Though President Barack Obama fell short of fulfilling his campaign pledge to end the war, his Administration vastly decreased troop levels from a high of more than 100,000 and shifted the remaining US forces to a less dangerous training and advisory role.
Inside the Administration, Vice-President Mike Pence and Defence Secretary Jim Mattis have advocated for the Pentagon plan to add troops, along with broader diplomatic and economic pressure on regional players, especially Pakistan.
"There's no easy answers in Afghanistan and no perfect solution," Andrew Wilder, an Afghanistan and Pakistan expert at the US Institute of Peace, said before the speech. "What I hope to hear tonight is not just a focus on troops but more importantly a political strategy guiding our security strategy. The key focus must be how do we shift from winning the war against the Taliban to winning the peace?"
Trump's task was magnified by his need to convince his core supporters, many of whom responded to his campaign calls to put "America first" by reducing foreign interventionism in the Middle East and Central Asia. His speech was taking place days after the departure from the White House of chief strategist Stephen Bannon, who had advocated for replacing US troops with private security contractors.
The President has framed his decisions to use military force - including Tomahawk missile strikes on an air base in Syria in April after President Bashar al-Assad used chemical weapons - around his pledge to be tough on terrorism and direct threats to American security.
White House press secretary Sarah Huckabee Sanders issued a statement noting the fourth anniversary of a previous chemical weapons attack by Assad. In that instance, Obama decided not to follow through on his pledge to strike the regime for violating a "red line".
"We must speak with one voice and act with one purpose to ensure that the Syrian regime, and any other actor contemplating the use of chemical weapons, understands that doing so will result in serious consequences," the statement said.
But foreign policy analysts said Trump's decision on Afghanistan is tricky because the strategy does not represent a radical departure from the past.
"To be honest, it's probably pretty close to what a Hillary Clinton would do," said Derek Chollet, an assistant secretary of defence in the Obama Administration who now serves as a defence analyst at the German Marshall Fund.
Chollet added that for a president who has measured success by decisive and dramatic action, the Afghanistan strategy "is about incremental success, if any. What's required in the speech is a president who is going to acknowledge why he's willing to take this risk . . . and asking the American people to have patience in a strategy that might not succeed".
Even before his presidential campaign, Trump was sceptical about the war. "When will we stop wasting our money on rebuilding Afghanistan?" he tweeted in 2011. "We must rebuild our country first."
As a candidate, Trump argued for a more isolationist approach to foreign policy. Recent foreign wars, he told his supporters, had drained America of blood and treasure at the expense of efforts such as education and infrastructure at home.
"So we're on track now to spend, listen to this, US$6 trillion - could have rebuilt our country twice - all together, on wars in Iraq and Afghanistan and the Middle East," Trump said during a speech in Cleveland during the campaign. "Meanwhile, massive portions of our country are in a state of total disrepair."
The argument was a departure from the Republican Party's hawkish stance on military engagement and the two post-9/11 conflicts in Afghanistan and Iraq that began under President George W. Bush. Trump's stance helped him appeal to some Democratic-leaning voters who were sceptical of the wars and perceived Clinton as more hawkish than many in her party.
Within the White House, Bannon's opposition to sending more troops to Afghanistan helped fuel strife with other Trump aides, including National Security Adviser H.R. McMaster, who supported the modest troop surge.
Bannon had advocated for a proposal to replace US troops with private security contractors, an idea floated by Erik Prince, the founder of Blackwater and brother of Education Secretary Betsy DeVos. Military leaders largely opposed the idea, and the White House ruled it out.
Analysts said Trump's decision to deliver his remarks in prime time at a military installation followed in the path of his predecessors, who used such settings to emphasise the gravitas of their decisions and help galvanise public support.
Yet they cautioned that Trump has shown a tendency to stray from prepared remarks and ad-lib more inflammatory statements aimed at pleasing his base. Bannon has returned to his previous post as the head of the conservative news site Breitbart, which could attack Trump's decision.
"This is Trump's war now," Chollet said. "Putting 50 per cent more troops in Afghanistan - that's ownership. And it's not something he can blame on his predecessor."
