A BBC correspondent said Mr Kiir, 63, looked well as he greeted other leaders and took his seat for the talks.
However, the re-emergence of a dispute between Mr Kiir and his South Sudanese rival, Riek Machar, caused substantive talks to be postponed until Saturday.
The discussions are aimed at ending the civil war in South Sudan.
The conflict, which began in 2013, has caused about 1.5 million people to be displaced.
Regional leaders are pushing Mr Kiir and rebel leader Mr Machar to agree a peace deal.
Earlier, it emerged that Mr Kiir had received medical treatment after suffering from nose bleeds on Wednesday, causing the intended start of the summit to be delayed.
Mr Kiir’s aides denied reports that the president had been taken to hospital on Wednesday night.
Mr Machar is also in Addis Ababa for the talks, which are being chaired by Ethiopia’s Prime Minister Hailemariam Desalegn.
Mr Kiir and Mr Machar met on Wednesday in the Ethiopian capital, in talks brokered by the regional body, Inter-Governmental Authority on Development (Igad).
The BBC’s Emmanuel Igunza, who is at the summit, says the two sides have previously agreed to form a unity government but remain deadlocked over the powers of a new prime minister.
President Kiir sacked Mr Machar as South Sudan’s deputy president in 2013 and later accused him of plotting a coup.
Mr Machar denied the allegation, but then raised a rebel force to fight government troops.
Mr Kiir has led South Sudan, the world’s newest state, since its independence from Sudan in 2011.
SOurce: Click Here