Jose Padilla had been expected to enter a plea on Friday on charges of plotting to "murder, kidnap and maim" abroad, but now must wait until 12 January.
He appeared in court in Miami after being transferred to civilian custody.
His case has huge legal and political significance, as it focuses on presidential power at a time of war.
Mr Padilla's detention, heavily criticised by civil rights groups, is seen as a test of the limits of the US government's anti-terrorism powers.
(More ...BBC NEWS | Americas | US terror suspect's plea delayed)