Joao Batista Groppo, 64, was arrested after his wife of 40 years, Sebastiana, was found confined in a "filthy, dark" cellar, said police inspector Jaqueline Barcelos Coutinho.
Groppo's girlfriend, Maria Furquim, was arrested as an accomplice by police in Sorocaba, about 50 miles (80 kilometers) west of Sao Paulo, the inspector said.
Initially, Groppo told police that he had locked his spouse up for 16 years. He later revised the time period to eight years, which their son confirmed, Coutinho said in a telephone interview.
The son told her he knew what his father was doing but was unable to persuade him to stop, Coutinho said, adding that the son "may face charges of failing to come to the aid of someone in need." She declined to identify him.
Groppo, who described himself as a retired industrial consultant, said he locked up his 64-year-old wife beginning in 2003 because she is mentally ill and aggressive, the inspector said.
"He told us that locking her up was the only way he could think of to prevent her from wandering off and getting lost," Coutinho said. "She does have psychiatric problems but she is definitely not an aggressive person."
Coutinho added that Groppo claimed he took his wife to visit a psychiatrist once a month.
Police were tipped off to the situation by an anonymous telephone call and arrested Groppo on Wednesday, the inspector said.
She said she was shocked when she arrived at Groppo's house and found the woman behind a padlocked iron gate in the cellar.
"She was lying nude on a concrete bed inside a foul-smelling, humid cubicle with no electricity or ventilation," Coutinho said. "The walls were covered with mold and spider webs. She was in a degrading situation unfit for animals."
Sebastiana was taken to a local hospital and released several hours later. She was then taken to her son's home in a nearby city, Coutinho said.
"She appears to be in good physical condition, but to be on the safe side we asked the son to make sure she gets a full checkup and continued psychiatric help," the inspector said.
Groppo and his girlfriend face charges of depriving someone of their freedom through "private incarceration" and could be sentenced to up to eight years in jail, Coutinho said.
Under Brazilian law, prosecutors only file formal charges after police finish their investigation.