"Mr. Bosco Ntaganda was transferred to... the Kingdom of Belgium to serve his sentence of imprisonment at the Leuze-en-Hainaut prison," the Hague-based ICC said in a statement.
"The ICC relies on the support from states for the enforcement of its sentences and is highly appreciative of the voluntary cooperation of the Belgian government in this case," the court's registrar Peter Lewis said.
Ntaganda, 49, dubbed the "Terminator", was convicted by the ICC in 2019 of leading a reign of terror in the northeastern Democratic Republic of Congo in the early 2000s.
The Rwandan-born Ntaganda was found guilty on five counts of crimes against humanity and 13 counts of war crimes, including murder, sexual slavery, rape and using child soldiers.
Ntaganda was the first person to be convicted of sexual slavery by the court. Many of the other charges related to massacres of villagers in the mineral rich Ituri region of DR Congo.
Prosecutors portrayed him as the ruthless leader of ethnic Tutsi revolts amid the civil wars that wracked the DRC after the 1994 genocide of Tutsis in neighboring Rwanda.
Formerly a Congolese army general, Ntaganda became a founding member of the M23 rebel group, which was eventually defeated by Congolese government forces in 2013.
Later that year he became the first-ever suspect to surrender to the ICC, when he walked into the US embassy in the Rwandan capital Kigali.
Ntaganda's sentence was upheld on appeal last year.