There was no unanimous verdict among the jury on whether to sentence Arias to death or life in prison for the first-degree murder of her lover, Travis Alexander.
In announcing the mistrial, Judge Sherry Stephens gave a heavy sigh and said: "This was not your typical trial. You were asked to perform some very difficult duties."
Deliberations began Tuesday after Arias pleaded for her life to the same jury that convicted her. The jury announced Wednesday it failed to reach a unanimous decision, but Stephens instructed jurors to keep trying.
A new panel likely will be brought in for the retrial, set for July 18, unless the prosecutor takes death off the table agrees to a life sentence.
Under Arizona law, a hung jury in the death penalty phase of a trial requires a new jury to be seated. If the second jury cannot reach a unanimous decision, the judge would then sentence Arias to spend her entire life in prison or be eligible for release after 25 years. The judge cannot sentence Arias to death.
Arias' statement to the jury Tuesday was an apparent recantation of her desire to die in a post-conviction interview.
"Longevity runs in my family, and I don't want to spend the rest of my natural life in one place," Arias told a Fox reporter in the interview. "I believe death is the ultimate freedom, and I'd rather have my freedom as soon as I can get it."
But she told the jury Tuesday that if given the chance, she would use her time positively in prison by donating her hair to cancer patients, helping establish prison recycling programs and designing T-shirts that would raise money for domestic violence victims.
She also asked the jury to spare her life for the sake of her family.
"I'm asking you to please, please don't do that to them. I've already hurt them so badly, along with so many other people," she said. "I want everyone's healing to begin, and I want everyone's pain to stop."
She said that her comments in her post-conviction interview were made without full consideration of the situation.
"Though I meant it, I lacked perspective. To me life in prison was the most unappealing outcome. ... But as I stand here now, I cannot in good conscience ask you to sentence me to death because of them," she said.
The Associated Press contributed to this report.