Montgomery County District Attorney Kevin Steele says he will retry the case.
Constand, now 44, initially went to police about a year after she said Cosby assaulted her, but a prosecutor declared her case too weak to bring charges.
Cosby has said any sexual activity was consensual.
A somber feeling overtook the courtroom as Judge Steven T. O'Neill polled the jury to make certain that all members were agreed that the impasse between them was insurmountable. I'm proud to have been able to represent Mr. Cosby.
Bill Cosby's trial has ended in a mistrial due to a hopelessly deadlocked jury but the circus that was his court case will be repeated, with prosecutors immediately announcing they will retryhim. Defense Attorney, Brian McMonagle also made a statement saying that Cosby became the focus of these accusations because he was an easy target for the woman who thought they could get money out of him.
Constand herself sued Cosby in 2005 and settled for an undisclosed sum.
Cosby's wife of almost 50 years, Camille, was seated in the courtroom on Monday for the closing remarks, and watched as the 79-year-old comedian's lead lawyer, Brian J. McMonagle, insisted that the alleged incident between Cosby and Constand was consensual and that their relationship was romantic in nature.
Cosby's trial, which lasted for about a week with a second week of deliberations, included 13 witnesses, including Constand - who was grilled for seven hours. Cosby is on trial for sexual assault.
The meaning of the phrase "without her knowledge" in one of the three counts against him.
O'Neill reminded Cosby, 79, that the three counts of aggravated indecent assault remain intact and that he will remain free on 10 percent of $1 million bail, pending the retrial, which some hinted may occur within 120 days. The Associated Press does not typically identify people who say they are victims of sexual assault unless they grant permission, which Constand has done. A subsequent investigation found insufficient evidence to pursue criminal charges against Cosby.
The jury began deliberating Cosby's fate late on June 12, following a six-day trial in Norristown, Pennsylvania.
Constand is the only one of the comedian's more than 40 accusers whose report has led to a criminal trial because in all of the other cases, the statute of limitations had already expired. By doing so, it "raised the question of whether some jurors are hoping to convince one or more others that any doubt they have expressed is unreasonable". Prosecutors now have four months to decide whether they want to retry the TV star or drop the charges.