The family told police the young woman had most likely been travelling with her French boyfriend, identified only as A. J. P. R. G. by Catalan daily La Vanguardia.
The family of the boyfriend also reported him as missing, while noting the girl's fragile psychological state, the daily added.
The boyfriend is currently being sought by French police after staff at the Olivia Plaza hotel in Barcelona's central Plaça Catalunya discovered the body of the young woman after entering the room at 12.30pm on Sunday.
The body reportedly showed signs of knife wounds and police said that they hope the missing man will be able to explain what had happened.
The knife used to inflict the injuries was found in the room and police are now studying hotel surveillance video as investigations continue.