The Space Command, or SpaceCom, is the military's newest and 11th unified combatant command, joining the ranks of U.S. Cyber Command and U.S. Strategic Command.
"The dangers to our country constantly evolve, and so must we," Trump said at a White House event, citing new technologies threatening American satellites and potential missile launches targeting the U.S. "Now those that wish to harm the United States, to seek to challenge us in the ultimate high ground of space -- it's going to be a whole different ball game."
The command is distinct from the "Space Force" military service that Trump has said he wants to create and from the existing Air Force Space Command. Congress would have to approve a new, sixth branch of the military, to which the president on Thursday reaffirmed his commitment.
At the White House ceremony, Secretary of Defense Mark Esper signed documents formally establishing the command, and a new SpaceCom flag featuring a bald eagle and the earth was unfurled.
The Army, Air Force, Navy and Marine Corps can all contribute personnel and assets to this new command, which will coordinate how the U.S. military operates in space.
It will change how the Department of Defense approaches war-fighting there by centralizing its space operations from across the U.S. military into one place, to counter threats from China and Russia. Four-star Air Force Gen. John Raymond will lead it; he has already been confirmed to the job and currently heads Air Force Space Command.
"SpaceCom will ensure that America's dominance in space is never questioned and never threatened," Trump said at the White House ceremony, which took place in the Rose Garden. Vice President Mike Pence and Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff, also attended.
The command is technically being reactivated. The command was stood up in 1985 but deactivated in 2002 after the Sept. 11, 2001, attacks led the military to focus more heavily on homeland defense and counter-terrorism.
"It shares the same name. But it's a different command built for a very different strategic environment," Raymond told reporters at the Pentagon on Thursday. It will have "a sharper focus on protecting and defending, with a sharper connection to our intelligence community, with a sharper connection to our allies and partners," he added.
It will start with 287 personnel who will come from Air Force Space Command and U.S. Strategic Command. They will have the goal of reaching initial operating capability as it brings in units under its command, with no estimate for how long that process will take.
There are six bases under consideration to house the command permanently.
ABC News' Luis Martinez contributed reporting to this article.