US Attorney Joyce White Vance said 25-year-old Jarvis Britton entered the guilty plea on Monday.
A federal grand jury had indicted Britton in September on charges that he knowingly threatening to kill, kidnap or inflict bodily harm on the president.
Prosecutors said he tweeted, "Let's kill the president. F.E.A.R.," on 14 September.
Several people, including soldiers from Georgia's Fort Stewart, have been connected to a national militia group that prosecutors say was called F.E.A.R. - Forever Enduring Always Ready.
They said the group had terrorism plans and an ultimate goal to overthrow the US government and assassinate the president.
Britton's sentencing is scheduled for 20 June.