According to AFP dispatch, Rapp will leave his current functions on 7 September.
Rapp replaces Clint Williamson.
Before his nomination to the SCSL, Rapp, 60, was Chief of Prosecutions at the Arusha-based International Criminal Tribunal for Rwanda (ICTR), trying key suspects of the 1994 genocide.
"One of my functions will be to ensure that the SCSL and the other international tribunals have the necessary resources to conclude their missions", Rapp was quoted to have said by AFP.
The SCSL in Freetown was set up in 2002 with the support of the U.N. to try the alleged authors of war crimes and crimes against humanity committed during the civil war which ravaged Sierra Leone from 1991 to 2001, resulting in 120,000 dead and thousands of people who were mutilated.
Since January 2008, former Liberian President Charles Taylor was tried for his alleged involvement in the conflict, in The Hague, Netherlands, where the trial was moved for safety reasons.
© Hirondelle News Agency