Michelle Martin, 52, has been in prison since 1996, and was convicted in 2004 of being Dutroux's accomplice in the deaths of two eight-year-old girls and jailed for life.
She is expected to enter the Sisters of Clare convent near Namur.
The abbess, Sr Christine, said the convent is acting "in concert with the law".
Victims' families are expected to mount a legal challenge.