The jury, which began deliberating at 6 p.m. ET on Tuesday, sent a note to Judge Sherry Stephens shortly before 3 p.m. ET today saying they could not come to an agreement.
They are tasked with deciding whether Arias will get the death penalty or a sentence of life in prison.
The same jury convicted Arias, 32, earlier this month of first-degree murder for killing her ex-boyfriend, Travis Alexander, in a bloody attack in 2008. Prosecutor Juan Martinez has argued that the cruelty of the murder warrants the death penalty.
Arias and her attorneys begged the jury on Tuesday to spare her life and sentence her to prison rather than death.
Arias, with her hair pulled back in a tight ponytail, listened calmly as Judge Sherry Stephens told the jury to go back to their deliberation room and try once again to come to an agreement this afternoon.
The family of Travis Alexander, seated in the gallery of the courtroom, began to cry as they heard the jury's note read aloud by Stephens.
"Each juror has a duty to consult with one another, (and) to try to reach agreement without violence to individual judgment. You may want to identify areas of agreement and disagreement. If you still disagree, you may wish to tell the attorneys and me what issues, questions or law or facts on which we can possibly help," Stephens told the jurors.
"At this time please go back to the jury room and continue deliberating," she said.
If the jury cannot agree on a sentence for Arias, new jurors will be selected and the penalty phase will begin again. If a second jury cannot decide unanimously on a penalty, Arias would be sentenced to either life in prison without parole, or life in prison with the possibility of parole after 25 years.
The jury currently deliberating has been listening to testimony in the case since the beginning of January.