Jozef Wesolowski, a Pole who was defrocked by a Vatican tribunal in June, was placed under house awaiting a criminal trial, the Vatican said in a statement.
Perhaps the arrest was in aid of staving off worse repercussions. Or perhaps it represented a more open and responsible approach to a problem the church has strained to evade.
Francis, the first non-European pope in 1,300 years, vowed zero tolerance against Roman Catholic clerics who sexually abused children.
Last May, he called such abuse an "ugly crime" and likened it to "a Satanic Mass".
In July, he told victims of sexual abuse the Church should "weep and make reparation" for crimes he said had taken on the dimensions of a sacrilegious cult.