Superior Court Judge John Wiley ordered today that Dawnette Knight, 32, be examined by Dr Kal Sharma by September 15.
The psychiatrist will determine whether Ms Knight is competent to stand trial and can work with her lawyer.
The examination will be conducted in jail, where Ms Knight is being held on $US1 million ($1.4 million) bail.
Ms Knight, who wrote numerous letters threatening the Oscar-winning actress, is charged with one felony count of stalking and 24 felony counts of making criminal threats.
A hearing to determine whether there is enough evidence to put Ms Knight on trial was suspended last month when defence lawyer Richard Herman requested the psychiatric exam after an incident in which he suspected his client attempted suicide.
Ms Knight was too drowsy for a court session in July and told her lawyer she had taken sleeping pills, Mr Herman said, but he suspected she tried to overdose on barbiturates.
Outside court today, Mr Herman repeated allegations that Zeta-Jones had driven his client to attempt suicide by orchestrating a media campaign against Ms Knight.
"It's being waged by Zeta-Jones with her knowledge and it's abusive," Mr Herman said. "... It's an abuse of her position and her power."
Mr Herman said his client was never a threat and never should have been arrested.
The defence lawyer said he believed Ms Knight would be found competent to stand trial and that she should receive probation and counselling rather than any prison time.
Zeta-Jones and her husband, Michael Douglas, testified about the letters at last month's preliminary hearing.
"We are going to slice her up like meat on a bone and feed her to the dogs," said one letter read in court by the actress.
Earlier this summer, Ms Knight apologised for writing the letters and claimed she had been infatuated with Douglas.