The woman, also thought to be from the UK, is understood to be in a critical condition in hospital in the city of Albufeira in the Algarve region.
A Foreign Office spokeswoman said: "We are aware of the hospitalisation of a British national on May 29 in Portugal. We are providing consular assistance."
The woman was stabbed several times and the suspect is due to appear in court today, according to a report from ITV News.
Albufeira, on the south coast of Portugal, is a popular resort for many Britons.