UN MINUSMA spokesman Olivier Salgado has been given 72 hours to leave over "tendentious and unacceptable" posts he made concerning a controversy involving Ivorian troops, it said in a statement received by AFP.
The expulsion comes amid mounting friction between Mali's ruling military and international partners supporting the country's fight against jihadists.
The issue over Salgado relates to 49 troops from Ivory Coast who were detained after landing at Bamako airport on July 10.
The authorities have accused the troops of being "mercenaries".
But Ivory Coast says they were sent to provide a support role for MINUSMA, under a routine rotation.
According to Mali's version of events, the troops had no mission orders or any authorization to enter the country.
The foreign ministry statement accused Salgado of Twitter posts "declaring without any proof that the Malian authorities had been previously informed" of their arrival.
"The ministry cannot remain passive in the face of this behaviour, which harms the partnership with MINUSMA and other partners," it said.
MINUSMA, it said, was asked to provide evidence to support Salgado's assertions but had given no reply.
MINUSMA -- the United Nations Multidimensional Integrated Stabilisation Mission in Mali -- was launched in 2013.
It is one of the UN's biggest peacekeeping operations, with 17,609 troops, police, civilians and volunteers deployed as of April, according to the mission's website.
It is also one of the most dangerous UN missions, with 275 fatalities from attacks, accidents or other causes, according to the website.
On Friday, Egypt, MINUSMA's biggest single troop contributor, said it would "temporarily suspend" its participation in operations after seven of its troops were killed this year.
Meanwhile, troops in France's Barkhane anti-jihadist mission in the Sahel are expected to complete a pullout from Mali in the coming weeks.