Mr Abdul Razak denied ordering the woman's murder
Malaysian political analyst Abdul Razak Baginda has been acquitted of involvement in the October 2006 murder of a Mongolian interpreter.
Two policemen will face trial on 10 November for the murder of Altantuya Shaariibuu, who was blown up with explosives after being shot.
Mr Abdul Razak has admitted to an affair with Ms Shaariibuu.
He is ex-aide to Deputy PM Najib Razak, whose alleged involvement has turned the murder into a political scandal.
"I just want to go home," Mr Abdul Razak, who is 48, told reporters after hugging his family.
At the time of his arrest, he was director of the Malaysian Strategic Research Centre, which was founded by Mr Najib in 1993.
High Court Judge Mohamad Zaki Yasin ruled that the prosecution had failed to establish a case against Mr Abdul Razak following a trial that began nearly two years ago.
The prosecution had tried to argue that he had ordered the woman's murder after her demands for money became oppressive.
But after the calling of 84 prosecution witnesses, the case had collapsed.
Mr Najib, who is expected to become prime minister in March next year, has firmly denied repeated allegations of any links to the case, other than friendship with Mr Abdul Razak.
Several mysteries related to the case remain.
The explosive allegedly used is described as "military-grade", and the two policemen still on trial were members of the elite Special Action Squad.
They are Chief Inspector Azilah Hadri and Corporal Sirul Azhar Umar.
A private detective, P. Balasubramaniam, had said in a statutory declaration that Mr Najib had also had an affair with Ms Shaariibuu. He altered the statement the next day and then went missing.
A popular blogger, Raja Petra Kamaruddin, made a statutory declaration and posted allegations linking the deputy prime minister to the case.
He is now behind bars under the Internal Security Act which allows for indefinite detention without trial.
Analysts say that the acquittal of Mr Abdul Razak could take some of the pressure off Mr Najib, although Malaysia's cyber-community is already discussing how much mud from the trial will stick to the politically powerful.
Ms Shaariibuu was a highly educated mother of two, who spoke Russian, English, Chinese and Japanese. Born in Mongolia, she was raised in St Petersburg and educated in Beijing.