Lead investigator Paulo Alberes told The Associated Press that authorities decided Gatti killed himself on July 11 while at a seaside resort in northeastern Brazil.
When asked whether police had determined the case was a suicide, Alberes said "yes." He offered no other details.
A day after the 37-year-old Gatti was found dead, police said that his Brazilian wife, Amanda Rodrigues, had strangled him with her purse strap as he drunkenly slept.
But police began to back off the accusation about a week later after a coroner's report said Gatti might have killed himself as he was found "suspended and hanged." The autopsy report didn't exclude the possibility he was slain, but is said he also could have died in an unexplained accident, besides killing himself.
"The police investigation concluded that Arturo killed himself," said Celio Avelino, Rodrigues' attorney. "I've said before it would have been impossible for her to suspend and hang a man of that size."
Judge Ildete Verissimo de Lima ordered the immediate release of the 23-year-old Rodrigues after receiving the police report.
The judge wrote in the ruling that police informed the court "the detention of the suspect was no longer needed" as the investigation "excludes the possibility of murder."
"The victim ... committed suicide by hanging," Lima's ruling read, referring to the police investigation findings.
Gatti, who captured two world titles in his 16-year pro career, retired in 2007 with a record of 40-9.
Gatti's family and friends in his adopted hometown of Montreal denied the boxer could have killed himself.
"I'm a little bit shocked, I can't believe it. I don't even know what to say," said Radka Baranaiova, the fiancee of Gatti's brother, Fabrizio.
At his July 20 funeral in Canada, others said there was no chance the Gatti -- known for his brutal fighting style and ability to take relentless punishment yet still pull out a victory -- would have killed himself.
"Nobody believes whatsoever that there's even a one percent chance of a suicide. He lived life to the fullest," Ivano Scarpa, a close Gatti family friend, said at the time.