The coup comes a month after the military seized power in another African nation, Niger.
The United States late Wednesday urged Gabon's military to preserve civilian rule, voicing concern after coup leaders ousted the entrenched president following disputed elections.
"We urge those responsible to release and ensure the safety of members of government and their families and to preserve civilian rule," State Department spokesman Matthew Miller said in a statement.
The statement, in line with language from Western allies, comes hours after the military placed under house arrest President Ali Bongo Ondimba, whose family has ruled the oil-rich African state for 55 years.
"The United States is deeply concerned by evolving events in Gabon. We remain strongly opposed to military seizures or unconstitutional transfers of power," Miller said.
The coup comes a month after the military seized power in another African nation, Niger.
Unlike Niger, where ousted president Mohamed Bazoum was a key Western ally praised for his democratic credentials and who brought in American and French forces to fight militias, the United States has limited engagement with either Bongo or Gabon's military.
The US statement voiced concerns over the "lack of transparency and reports of irregularities surrounding the election", which the opposition claimed to have won.