The 68-year-old man allegedly fired three shots at a 41-year-old woman, hitting her in the head, the PJ said in a statement.
According to the police, the crime occurred at 8pm on Monday, "in the middle of the public road in the city of Portimão".
The man was later arrested by the Criminal Investigation Department (DIC) of that city's PJ, in collaboration with the PSP.
The detainee, suspected of a crime of murder in the form attempted, will be present in court to be heard in the first judicial questioning.
Lusa tried to obtain more information about the case from the PJ, but it has not been possible so far.