Awow Daniel Chuong, who was just four months into the job, state-owned television reported.
According to the state-owned television station, this is the sixth replacement in the post since 2020.
Kiir gave no reason for firing Mr Chuong, who was appointed in mid-March this year, the report said late on Wednesday, and economist Marial Deng has been tipped to replace him as finance minister.
South Sudan’s economy has been under pressure in recent years amid communal violence, with crude oil export revenue having dwindled since a 2013-2018 civil war and more recently export disruptions due to war in neighbouring Sudan.
Central bank governor James Alic Garang said in May that South Sudan’s foreign exchange reserves were at historic lows.
The International Monetary Fund (IMF) projects consumer price inflation at a lofty 54.8% this year.
Kiir became South Sudan’s first president in 2011 when it gained independence from Sudan.
The country is due to elect a president, members of parliament and regional representatives in December.
