Thomas Lubanga was convicted in March of recruiting and using children in his Union of Congolese Patriots militia, sending them to kill and be killed during fighting in Congo’s eastern Ituri region in 2002-2003.
“The vulnerability of children means they need to be afforded particular protection,” he said at the sentencing hearing.
The 51-year-old Lubanga is the first person convicted by the 10-year-old permanent war crimes tribunal.
Rights activists hailed the case as a milestone in efforts to prosecute the widespread use of child soldiers in conflicts around the world.
Prosecutors had asked for a 30-year sentence, but said they would be willing to cut it to 20 years if Lubanga offered a “genuine apology” to victims of his crimes.