Police believe the head, found in a suitcase, is likely to belong to a 27-year-old Japanese woman who went missing after visiting another apartment in western Osaka prefecture with the man earlier this month, according to public broadcaster NHK.
A local police spokesman declined to confirm the case.
The woman was last seen walking with the suspect, a 26-year-old man from New York, who was arrested last week after she was reported missing by her family.
He has reportedly denied the allegations.
The woman had told her friends that she was visiting Osaka to meet a man she had met on an app.
Local media reported that video footage from a surveillance camera showed the woman entering the condominium with the man, before the man was later seen coming in and out alone several times, carrying a large bag.
The man had arrived as a tourist in Osaka in January, the Asahi Shimbun daily reported.
Body parts other than the head have not been found.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)