The announcement was made during a televised speech by Thai Army chief Prayuth Chan-ocha, who was flanked by senior army officials during the telecast. Political officers who were attending a meeting called by the military had been detained.
The coup follows the introduction of martial law in the country Tuesday.
"The national peacekeeping committee," Prayuth said, referring to the military, "will worship and protect the monarchy."
Leaders of both political parties were seen being taken away in vans at the Army Club in Bangkok, where they were meeting to try and resolve the political deadlock in the country, which has led to violent unrest at times.
Prayuth said this was a full blown coup, and that the army, navy, air force and police had taken control of the country to quell the political violence and to ensure fairness to every side.
"I ask the people to remain calm and carry on with their business as usual," he said.
Thailand has a long history of coups, with this being the 19th coup it has faced since 1932. Protestors have been fighting to rid the country of the Shinawatra family, Thailand's most powerful political family.
Thaksin Shinawatra, the patriarch of the family, was removed from power in 2006 by the military. Thaksin established the Shinawatra family during a populist movement in 2001, and his family has won every election since then.
Prayuth gave no indication of whether fresh elections would be held anytime soon.