Explaining the move, the Pentagon on Wednesday said: "The president determined that there is reason to believe that each of these enemy combatants was a member of al-Qaida or was otherwise involved in terrorism directed against the United States."
No charges have been approved against any of the nine, who were not identified.
That brought to 15 the number of non-US citizens held at the US naval base at Guantanamo Bay, Cuba, as "unlawful enemy combatants" whom Bush has deemed eligible for such trials.
These would be the first of their kind since the second world war.
Three others were charged earlier, and Pentagon officials have said they hope to conduct a trial by the end of the year.
Human rights groups have sharply criticised the rules created by the Pentagon for the trials.
The decision comes as the Bush administration struggles to respond to Supreme Court rulings last week that affirmed the right of detainees at the detention centre in to challenge their incarceration in US federal courts.