The woman disappeared from her home in central Timrå last weekend.
A week ago she was reported missing by her family.
When tracking the woman's mobile phone, there were several hits in the Kramfor area during the last week.
Late on Friday, the man who has now been remanded in custody was arrested and detained.
Then as reasonably suspected (the lower level of suspicion) of kidnapping.
During the weekend's investigative work, findings were made that led to District Prosecutor Christina Edlund Nilsson sharpening the crime classification to murder and kidnapping.
In both cases on probable cause, which is the stronger degree of suspicion.
The man and the woman knew each other from before, the police confirm.
No further comments
It appears from the arrest warrant that the suspicion of kidnapping concerns the woman's home address in Timrå and the suspicion of murder is linked to a place west of Kramfors.
The man who was arrested and detained late on Friday denies any crime.
His public defender, Sundsvall's lawyer Per Näslund, was banned from speaking in the case by the prosecutor on Monday and is therefore not allowed to give any comments at all.
The suspect is not previously known to the police and has not been convicted of any serious crime.
The detention hearing takes place in Sundsvall's District Court, tentatively during Tuesday morning.