The jury said in a statement, "Our special prize goes to the film that cannot be compared to any other in the competition".
"To the very radical, courageous, artistic and existential statement Mariupolis 2. Director Mantas Kvedaravicius is one of the thousands of civilians killed by the Russian army since the beginning of Putin's large-scale invasion."
The documentary jury was made up of Polish director Agnieszka Holland, Ukrainian writer Iryna Cilyk, French actor Pierre Deladonchamps, French journalist Alex Vicente and Moroccan filmmaker Hicham Falah.
Mariupolis 2 premiered at the official special screenings program Cannes Premieres.
The 45-year-old Lithuanian filmmaker and cultural anthropologist was killed in the Ukrainian city of Mariupol, which had already been surrounded when he arrived, in early April.