“I can confirm to you that our chairman has arrived in Addis Ababa airport this morning,” Lam Paul Gabriel said on Wednesday.
Machar had been held under house arrest in South Africa since late 2016 after fleeing South Sudan.
An Ethiopian government official also confirmed Machar’s arrival and said he would meet Ethiopia’s prime minister, Abiy Ahmed, before his talks with Kiir.
“Riek Machar has arrived,” Meles Alem, foreign ministry spokesman, told Reuters news agency.
Kiir and Machar are due to meet later on Wednesday. It will be the first between the two since a peace deal between the government and Machar’s rebel group fell apart in August 2016.
Tens of thousands of people have been killed in the civil war that broke out in late 2013 when troops loyal to Machar, the former vice president of South Sudan, launched a rebellion against the government.
The United Nations and other organisations have accused all sides in the complex and multifaceted war of committing atrocities against civilians.