Vitaly Churkin died suddenly at work in his New York office last month.
The office of the chief medical examiner said that details of his death were being withheld to comply with international law.
Mr Churkin had served as ambassador to the UN since 2006.
The cause of the veteran diplomat’s death remains unclear.
“The New York City Law Department has instructed the Office of Chief Medical Examiner to not publicly disclose the cause and manner of death of Ambassador Vitaly Churkin,” a spokeswoman for the medical examiner’s office, Julie Bolcer, said in a statement.
A state department official told the BBC that further comment or details on the results of the post-mortem could not be disclosed because of an agreement between the US and the UN.