The corresponding document was published on the website of the Ukrainian president.
"<...> I decide: to declare September 26 as a day of mourning in Ukraine," the message says.
The state flag will be lowered on the buildings of the authorities in the country.
All entertainment events will also be canceled, and corresponding changes will be made to the programs of TV channels and radio stations.
According to the State Emergencies Service, there were 27 people on board the aircraft, of which 23 people were declared dead, one was injured.
The search is underway for three more people.