Kent Police, the Foreign, Commonwealth and Development Office (FCDO) and Interpol are working together to help find her.
Paul Arnott, who helps find missing British nationals, wrote on social media: “Lucy had travelled to France for a music festival in the city of Brest and is believed to have subsequently been in Paris. No one has heard from her for the past week.
“Lucy is considered vulnerable, and efforts to locate her are ongoing. The British embassy, police authorities and Interpol are involved in the search.
“If you know Lucy, have spoken to her, seen her, or have any information that could help establish her whereabouts, please come forward as soon as possible.”
An FCDO spokesman said: “We are supporting the family of a British national who has been reported missing in France and are in contact with the local authorities.”
A spokesman for Kent Police said: “Kent Police is liaising with partner agencies, including the FCDO and Interpol, following the reported disappearance of Lucy Stemp, 28, from Tonbridge, in Bercy, Paris.”