Debra Milke was found guilty in Arizona state of the 1989 shooting of her four-year-old boy, who was allegedly killed for a $5,000 (Â£3,200) insurance payout.
Milke, 49, allegedly confessed to a detective whose credibility has since been questioned.
She will remain on bail as prosecutors mount a new trial.
Milke, then a 25-year-old insurance company clerk, was accused of dressing her son in his favourite outfit and telling him he was going to see Santa Claus at the mall.
She then allegedly handed him over to two men who took the boy to the desert and shot him.
Roger Scott and James Styers were both convicted in the case, but neither testified at Milke's trial.
A court of appeals overturned her conviction in March this year after questions were raised about a now-retired detective who testified that Milke had confessed the crime to him.
He was found to have lied under oath in four later cases and was suspended for accepting sexual favours on the job.
Milke's $250,000 bail bond was paid out of money raised by her European supporters. Her German mother has maintained her daughter's innocence.
Donors in Switzerland, where Milke's mother now lives, were reportedly spurred to contribute by doubts over her guilt and opposition to the death penalty.
Arizona Mark Milke, ex-husband of the accused, told the Associated Press on Friday he believes his former wife is guilty of the crime.
Prosecutors plan to seek the death penalty again at Milke's retrial, currently set for 30 September.
A US woman who spent more than two decades on death row after being convicted of murdering her son has been released because of a lack of evidence.