Lars Ånestrand of the Katrineholm police told news agency TT that the man is dangerous and could get even more violent if he drinks alcohol or takes drugs.
The man was convicted of rape in 1987. During the 26 intervening years, he has received treatment at different clinics and has committed more crimes.
Police fear that the mean left Katrineholm by bus or train.
"If you see someone who matches his description, you should call the 112 emergency number. You should not intervene yourself," Jocke Andersson of the Sörmland police told TT.
The man is 54 years old, has a deep cut in his right arm and a flag tattooed on his left shoulder.