The Civilian government of Alhaji Abubakar Tafawa Balewa was ousted in a bloody coup led by major Chukwuma Nzeogwu in January 15, 1966. This first Military Coup in Nigeria then brought in Major General J.T.U Aguiyi Ironsi as the first Military Head of State of Nigeria.
On 29th day of January 1966, the Military Government was overthrown in a bloody coup staged by Northern Regional Army Officers led by Lt Col. Yakubo Gowon.
On July 29, 1975, the Miliatry government of General Yakubo Gowon was overthrown by General Murtala Muhammed in a bloodless coup. The coup took place while he, General Gowon was attending the O.A.U. summit at Kampala Uganda.
An aborted military coup was staged by Lt. Col. D.S.Dimka
Dinka on February 13, 1976. General Murtala Muhammed was killed in the exercise. General Olusegun Obasnjo then became the military head of state. On the 1st day of October, 1979, General Obasanjo handed over the mantle of leadership to Alhaji Shehu Shagari, the first Executive President of Nigeria.
On December, 31,1983, the civilian government of Shehu Shagari was overthrown by General Muhammed Buhari and Tunde Idiagbon when they were at Mecca.
So the two military coups that toppled civilian regimes in Nigeria were those of January 1966 and December 1983.