The man, who had been held at a facility in Huddinge, was spotted in Skogås.
“An off-duty policeman saw him on the street and recognized him. He has now been driven back to the hospital,” said Mats Nylén, spokesman for Södertörn police.
The man had called police on Monday, telling them he would return to hospital on Tuesday, but he never turned up.
The 27-year old has been convicted several times of sexual offences against both children and women. Last March he was convicted of sexually assaulting a 5-year old girl, and was ordered to be held in a secure hospital.