Rakhat Aliyev, the former son-of-law of Kazakhstan's president, was found hanged in his prison cell in February 2015 just before he was due to testify in an extortion trial.
Officials at the time said the wealthy ex-diplomat, who was also facing murder charges in a separate case, had committed suicide.
But German forensic pathologist Bernd Brinkmann, who had been hired by Aliyev's widow, in December claimed that the businessman had been murdered by suffocation.
The prosecutor's office in Vienna has now rejected Brinkmann's findings after receiving a second expert opinion.
"The conclusion whereby Rakhat Aliyev was murdered cannot be forensically confirmed after extensive examination," it said in a statement.
The review of evidence showed that his death "can be consistently attributed to a suicide by hanging," Nina Bussek, a spokeswoman for the prosecutor's office, said.
Aliyev had made a name for himself after marrying the eldest daughter of Kazakhstan's autocratic leader Nursultan Nazarbayev in the 1980s.
He fell out with Nazarbayev in 2007 after reportedly revealing his presidential ambitions, was stripped of his post as ambassador to Vienna and ended up divorced.
A year later, a Kazakh court found him guilty of killing two bankers, who disappeared in 2007 and whose bodies would be found in 2011.
Aliyev eventually turned himself in to the Austrian authorities in 2014 over the case. Denying the murder charges, he was due to go on trial in April 2015.
The 52-year-old died just before his scheduled testimony in a separate trial of two fellow inmates who allegedly had threatened to kill him and make it look like suicide unless he paid them money.
Aliyev often talked about his fear of being murdered, even after he stopped sharing a cell with other inmates, a police officer testified at the extortion trial last year.